Boy oh boy, let's see what we have:
On a note: your alien bats, given their size, are probably better off being compared to flying foxes and other megabats in terms of how they'd work, and I plan to do just that. I will however also cheat a little and compare them to some microbat species if it helps justifying their traits.
As far as you I see the animals that share the most common traits with our niywits are clearly bats, so let's start by order from what's least problematic to what's most:
Eat insects and small berries primarily
No big deal, generalist bats like the flying fox are known to eat fruits, nectar, flowers and insects, which already shows its possible to be capable of digesting both. As for meat and eggs, the former is not that hard to believe, since we have carnivorous bats like the spectral bad (Vampyrum spectrum), eggs on the other hand isn't something I've ever heard about regarding what's in a bat's diet, but given that carnivorous animals can apparently digest them and I've found nothing on their consumption in itself being harmful (so long as you don't give them too many) I'd say it's believable enough. It'd probably be better for them to eat larger fruits other than just small berries, since larger bats on earth usually seem to have a diet a bit more reliant on fruits and other sugary foodsources for energy to survive.
Typically blue, steely grey, and black, with short fur (Abroad, they're bred to have longer fur and different colors)
The bluish fur is the trickiest one, since blue is not a color you find all that often in mammals. The rest however is totally believable, since colors found in earth bats include white, red, yellowish tones, orange, gray, brown and black so it's totally believable you could have other species with different colors (and yes, surprisingly a bright orange bat with black stripes is easier to find than a bat with bluish fur, like you see in the orange painted bat).
The face area (From in front of the ears, to chin, in a circle), has no fur, and the wing membranes also have no fur. They have blue, yellow, or green eyes.
Lack of fur in these regions? Little problem. The eyes are a little more tricky, since most bat species have black or brown eyes, with the flying foxes having golden colored eyes. It's not impossible, since he have other instances of mammals which possesses said colors, but in bats at least it's not really normal.
Sexually mature at 3 years old (Niywit equivalent to an 18-year-old Aerithean), live up to 12-14 years old (Roughly 72-84 year-old Aerithean), but can only breed up to 9 years of age (54-year-old Aerithean), with actively breeding males and females ending their breeding careers a year-ish earlier.
I'd suggest you give them the ability to have babies a little earlier, but it's still believable enough. Flying foxes life up to around 15 years and become sexual mature at 2 years old, with the big difference that females remain fertile from the first 12 to 13 years of her life. I'd advise you to follow a similar path, making them able to reproduce a little earlier and taking their ability away a little later.
Females have a single litter every year, gestating for 24 days (3 weeks Damarian time, 3 1/2 weeks Earth time), giving birth to 1 or 2 nibs/pups (The term for baby Niywits), nursing them for the first 9 months of their lives.
It's believable enough, since 1 is usually the norm but we have cases of bat species that can have up to 3 or 4 per gestation, so no big deal.
Niywits are polygamous, in the sense that they do not take a permanent mate; Males only stay with one female for breeding long enough to conceive offspring, then the female rears the nibs with other females.
No problem with this. Few bat species are monogamous and we have instances of bat species that actually live in colonies in which various females will help to raise the cubs. In vampire bat colonies we even have cases of females that adopted orphaned cubs after the mother's death (and yet some people still think they're just heartless bloodsuckers).
Niywits are social creatures, preferring to live in groups numbering anywhere from 3 to about 12 Niywits.
Are they not into parties?. Flying fox colonies can reach up to 15 thousand individuals, and having a diet mostly based on plant matter (and cicadas during certain parts of the year), there's still enough food for everyone. Your bats are apparently not into forming colonies it'd initially seem.
Good color vision (Similar to humans), good smell (Like a cat)
Again little problem, flying foxes already got both good vision and good smell, which they use to find one of their favorite food sources: fruit,which is also easy to hunt since it doesn't give much Chase.
They have a long, furry tail
A little weird for a large bat species to have a tail already, especially since they want to minimize body weight for flying and they already have some extra things weighting them down, which I'll get to in a moment. Normally I'd say it's better to not have it but I'll elaborate on the rest of the tail's traits later...
Now, for the trickier parts:
Bioluminescent in the wings (Along the bones in rows of dots, and running halfway up the wing in the shape of the trailing edge in rows of dots), face (Spots on the eyes and outsides of the ears), and hidden in the fur as stripes (Makes cute green stripes on the Niywit).
Bioluminescence is anything but something normally found in land animals on earth. It's a trait much more common in water, particularly in darker parts of the ocean in which light is useful for communication and predation, although we do have bioluminescent algae. Honestly I see no way to explain why they'd have bioluminescent wings, it sounds just too out of place, especially given other traits you want them to have.
They have four legs and a pair of wings, and walk on their legs (A type of hexapedal limb positioning called Western Draconic (Made it up)). Most land vertebrates on Damarian have 3 pairs of limbs, or evolved from something that did, so this isn't rare. Niywits walk well, and fly better than Earth bats
Its not impossible to walk well using limbs converted into wings you know? Vampire bats, despite their several adaptations for flight, are also skilled climbers, jumpers and runners, something they achieved by having stronger hind limbs, tightly folding wings and more developed padded thumbs for ground locomotion. You also say your bats can walk well and fly better than earth bats, which sounds a little weird, since unlike earth bats of similar size, yours are being weighted down by an extra pair of limbs, which you apparently imply are well developed for ground locomotion, and a tail. Unless it's easier to fly on your planet, I'd say they'll have a harder time to take off and stay airborne due to the extra "pointless weight". If you want your bats to be good at ground locomotion and at flying, I'd suggest your bats go through the following path:
1- Adapt the wings so they have strong thumbs for walking on the ground, and strengthen their hind limbs, this way you'll already be reusing the powerful wing muscles for the task of walking, reducing weight. At the end of the process they'd probably look more like a vampire bat in terms of stance:
2-reduce or just atrophy the 3rd pair of limbs. It's not ludicrous that a hexapodal animal would not loose a pair of limbs, but if pterosaurs are anything to go by, if you want to grow bigger without sacrificing flight ability, the key strategy is to Ditch anything that can be ditched as useless enough weight. If you really want them to have 6 limbs, reduce the middle pair so they're more useful at helping with with climbing or holding their young while flying, having the in between the first and last pair, usually tucked near the abdomen when not in use like in the following poor sketch:
That way you minimize weight by having them be smaller but allow them to still have a function: holding things like a squirrel while the bat flies, enabling them to manipulate and grasp food while using the other limbs to hold onto a tree or to walk around.
when owned as pets, much prefer to be carried in pockets or purses
A little weird, but younger flying foxes are usually left wrapped in blankets in animal hospitals and don't seem to complain much, so it might not be too crazy.
They have echolocation, which allows them to navigate in the dark, and can create audible noises (For Damarian sapients).
This bit, plus the hearing part, is when we start reaching problems. In nature at least, flying foxes can not use echolocation which is one of the reasons they rely so much on good sight and smell to find food. While we do have instances of creatures which have good sight, smell and hearing (like certain microbats), your bats, given their size, would seem better suited as generalist with a diet mostly based on fruits than an active insect eater (unless these insects are big and very nutritious, your bats would probably have a hard time relying mostly on those to survive, while a fruit diet sounds more believable given its size and other similarities with large fruit eating bats from earth).
Have camouflagable skin, which, combined with their ability to control the color and brightness of bioluminescence, creates a somewhat hypnotic effect
This trait makes a few contradictions clear. Your bat should be able to hunt at night, yet is bioluminescent wings would already leave it at a disadvantage since they'd work as a glowing alert to potential prey meaning they have little reason to use it while hunting. In addition their camouflageable skin enters in direct contrast with the bioluminescent stripes in their body and with their fur, because one key trait seen in the skin of squids, octopi and cuttlefish (and also in chameleons) is the absence of colorful things like fur that would hinder or completely invalidate the camouflage effect. Furthermore, even predatory birds and bats that surprise their prey usually don't use this trait, since a camouflage like seen in cuttlefish and octopuses is more associated with lie and wait predators (which completely invalidates strategies that involve the bat's main thing: expertly flying around) or communication with others of its kind (which doesn't make much sense in a creature with both great hearing and vocal cords that can echolocate and therefore has the additional advantage of not needing to be in the visual field of whomever they want to talk to in order to communicate). This combination of traits simply don't make much sense. If they have well developed hearing and echolocation, they don't need to use lights or colors to communicate like abyssal marine species and cephalopods do, and even if they can't use it, fruit bats can already communicate just fine with one another, whether it's night or day.
I understand they're mostly there for their special ability, but regarding why a bat would develop bioluminescence and camouflage: they probably wouldn't, because those traits are not necessary for communication if you have ears and vocal cords like an echolocating land animal normally would and because they're costly to maintain with little to no real advantage for a flying creature that hunts insects and eats fruits (and yes, I focused mostly on Cephalopods, because it's now known that chameleons use their skin color changing ability mostly to express their mood and communicate rather than to actually blend in with their surroundings).
And lastly, their tail and trademark attack:
They have a long, furry tail, which has a bulb at the tip, which generates a powder made of a chemical that, when ingested, is harmless, but when inhaled, creates effects similar to LSD (But to a smaller effect). The chemical is crystallized, then powdered, within the bulb, and the powder is capable to carry on the wind, which allows a Niywit to use it as an escape gadget (Spray the powder at a predator, then fly away while the predator's still high).
At least in humans, LSD is known to cause both hallucinations and wild mood swings as a symptom, which, if directly translated into the bat's predator, is very bad, because rather than something that has been discouraged to hunt you, you might have created something that's initially very aggressive and feeling like their lives depend on catching you,which is a potential side effect that could totally be avoided by simply spraying them with a foul smelling chemical produced at and released by the bat's rear region.
Furthermore, at least as far as I've found, while we have instances of animals that can cause psychoactive effects, these effects usually come from the consumption of (or at least attempt to consume) said animals rather than as a proper spray or powdered method of defense. As for a "powered" defense method released by an animal that can be carried by wind, we have tarantulas, which can release tiny urticaring setae from their bodies, causing irritation and discouraging a potential predator or threat, especially if it's a vertebrate.
So in the end, not only does the tail itself already acts as an extra weight to the animal, but I definitely see a risk, depending on how their psychoactive chemicals affect the potential predator.
As for using it as a collective defense mechanism, why? As far as I see they'd be trying to intimidate the would be threat and actually cause it psychological damage, which sounds valid enough until you remember we're talking about a group of flying animals that would be better off simply flying away, especially since earth bats are pretty good at making tight aerial maneuvers which increases their ability to dodge attacks thanks to their highly flexible and articulated wings, so the bats would be wasting time, energy and resources. Intimidation tactics and the use of superb camouflage are valid strategies seen in nature, but neither of those are all that necessary if you can fly. If you really want them to have a chemical attack and camouflage, it'd be better to have their fur be more similar in color and patterns to their habitat and add simple anal scent glands, or a shorter, lighter tail covered with special urticating hairs.
So finally, answering your main questions more directly:
How would a chemical like a weak LSD be able to be produced naturally (LSD is actually made from a fungus that grows on rye), become a powder, and be released to make a predator high, but be totally useless against young children?
I couldn't find any instance of such a thing occurring in nature that didn't really require the animal receiving a bite, problem is that when it comes to bats, one well landed bite from a predator can be enough to kill or incapacitate the animal, especially since flying foxes are predated by pythons, owls, eagles and some other birds, which means that at times the predator would only really get high after bringing the dead bat to their nest, if we're talking about the presence of other bird-like predators to your bat. Even if the bat had a colony of psychoactive fungi occupying its tail, it'd still probably require the predator to bite said tail.
Why would a bat-like creature have both bioluminescence and chameleon-like camouflage?
Chances are that they wouldn't. Even without a particularly well developed sense of hearing like seen in microbats sound is better than light or color to communicate because communication can be maintained without the need for both animals to be within visual range, and tactics that usually make use of light as a lure for insects that are attracted to light (like in abyssal fish like the famous anglerfish) or as a means to ambush prey (like in octopuses or cuttlefish) are just not all that great for a flying animal since both rely, to an extent, on the animal remaining still and waiting for prey to get close enough.
Could they ever be domesticated, if only for companionship?
Maybe. Truth is that, like I've said in my comment, bats are not really good pets and require a lot of care just so that they don't live less in captivity, which is common. Flying foxes are also in this list, requiring a lot of space to live and fly, good flow of air, a proper diet, a good number of friends (the "colonies of up to 1 5000 individuals" part also shows that bats are very social animals that usually need to have friends around, with flying foxes usually needing at least 9 more friends to feel comfortable, as far as I found) as well as other conditions to ensure they can live a healthy life free of unnecessary stress. It's probably not impossible to create sanctuaries in which the bats became accustomed enough to the sapient population, but I don't really see them acting like dogs or even as some birds. It's hard to say it's impossible with certainty, but by earth standards, bats are incredible, but very troublesome pets in terms of what it takes to keep them healthy and happy, and it'd probably take a lot to properly turn them into a domesticated species like cats or dogs.
Update: regarding your affirmation about everyone forgetting you're talking about creatures in an alien planet where magic exists and bioluminescence is something several creatures seemingly evolved: we're not, we're simply leaving that information aside for the moment (particularly the bioluminescence part, since you didn't mention it was common among several creatures like you did with it having 6 limbs, simply asking whether it was plausible for a bat like terrestrial creature to evolve such a trait) and explaining why certain traits are or aren't plausible based on earth standards alone (because technically by your fictional world's standards, whether a creature's traits are or not plausible is solely dependent on whether you want them or not to have said traits).