I've been working on an alien species for some time that combines several traits I find interesting into a single being. This species is for use in a fantasy story, but I want it to have a hard science anatomical structure so that I could use it in future sci fi stories as well. I will bold the areas where I need the most help in to make it at least slightly scientifically viable.
Basic Description: A semi-aquatic arboreal species of sexapods with digitigrade back legs and four arms. Look reptilian but are probably closer to a dinosaur biologically.
Habitat: They originated in a dense jungle dotted with swamps and rivers. The trees are very tall and thick, and have a wide canopy that covers the watery ground floor. There are large patches of dry land, but most of the area is wet.
Evolutionary Path: They evolved from a fresh water ambush predator much like a crocodile that's main method of attack was leaping out of the water and dragging it's prey down with it's arms. It eventually started climbing and ended up filling a different sort of predatory niche involving traveling through the trees and dropping down on unsuspecting prey, grappling them into the water.
Advanced Description: This species, on average, stand to be 6'5" to 7' tall on their hind legs and have six limbs. Certain members of the species grow much larger than that, but that will be covered below. They have a very slender, elongated build that minimizes bulk in favor of a lighter weight and more efficient strength. A set of arms coming from the torso and a pair of digitigrade legs, along with a long, prehensile tail that is roughly the length of the full body from head to toe. The upper set of arms are slightly longer than the legs in proportion while the lower set of arms are closer to what humans would consider proportionate to it's size. It's body is covered in a thin layer of scaled, leathery skin that is highly flexible and smooth, much like the skin of a snake. The color of the scales can change, largely in response to the creature's mood though they have some conscious control over the changing color as well. Each limb ends in three digits (Though I am debating on the number of fingers and toes) ended in slightly curved claws with dull edges and sharp points. The head has a slightly pronounced jaw-line designed to allow for superior bite force, but not so pronounced that it can't support lips for better vocalization. I am also debating on giving them small conal horns that aid in channeling sound waves so compensate for not having proper ears. For hair they have long, very thin feathers that grow from their cranium like a crest. It would appear like hair to anyone looking at them from a distance, and culturally is treated much like how we'd treat our hair. Ideally the feathers are water resistant in some way so that they do not become a burden to the creature when submerged or in the rain.
Breathing: I've done a lot of research into why a four armed creature can't breath, but I didn't find many work arounds in it's place so I started looking through the animal kingdom. The best answer I could find were turtles. Like turtles this creature would likely need sheets of muscle tissue around their lungs that constantly expand and contract around the lungs, allowing the creature to breath despite the chest being contained and unable to expand with it, similar to a turtle shell. It isn't as energy efficient as normal breathing of course, but giving the creature generally more efficient lungs and blood to make oxygenating the muscles more efficient can compensate a bit. This does still leave it with a loss of upper back mobility though, and a thought I had that I can't find much information about was that it's solar plexus is broken into flat vertibrae similar to a second spine. This would theoretically make the creautre able to bend it's upper back at least slightly at the cost of a weaker chest. I am unsure how scientifically valid this concept it though. Naturally as a semi-aquatic species it's muscles are very efficient at storing oxygen for later use, allowing it to stay submerged for about an hour at a time. It may also be able to absorb oxygen from the water through it's butt, much like the beloved turtles I referenced earlier.
Body Temperature: I was divided between warm blooded and cold blooded for this species, but came to a middle ground. The premise being that they are endotherms with the capacity to lower their metabolism and shut down the ability to produce body heat during periods of time when intense physical activity isn't necessary. So when it's warm out and there isn't a lot of hunting to be done they would convert to an ectothermic lifestyle, but when it gets colder or food is desperately needed they could convert back to endothermic. There should probably be some sort of trade off for this, but I can't think of one aside from it maybe using up a lot of energy?
Substantial Growth: A unique attribute of the generally larger male half of the species is the capacity to grow indefinitely. Females may have this ability too but just don't for cultural reasons. Basically if one of these creatures is consistently overfed it will continue to grow larger like a snake in a large tank. In this way the normally 7' tall male could grow upward 10-12 feet tall, though at that point it wouldn't be able to stand bipedally anymore and would likely not be able to effectively climb, sacrificing it's ability to hunt for itself in favor of being large and in charge. Females and beta males would do the hunting while this giant alpha fought off other large predators. Fights between tribes would usually be settled by a fight between alphas, and in cases of all out war the smaller female could ride her mate into battle in the form of a cavalry archer. They spend most of their time in the water when they can so as to minimize energy usage between fights. Becoming this large takes years and is a long term goal for any male leader of a tribe. Being so large does shorten their lifespan however, much as it does in abnormally large humans. Males that don't want to become giant need only not over eat, as they can only get bigger by overfeeding. With a stable intake of food they won't grow far beyond the 7' tall limit. Alternatively I had considered frequent mating with multiple female partners to be what triggered the capacity to grow. A sort of hormonal shift. If this idea is even possible I'd like some input on which method of beginning the transformation makes the most sense.
Green Blood: Their blood is actually white, which I read somewhere is the most efficient in terms of oxygen transportation. However it also has a high concentration of biliverdin that gives it a green pigment, much like a specific group of skinks on Earth. It doesn't have to be biliverdin though, and if someone could tell me a better way to get green blood that would be very nice.
Can use a Bow and Arrow: I kind of fell in love with the idea of a four armed archer. Not to hold and fire two bows at once or anything, but simply for the ease at which it could reload and fire rapidly. I know the claws would be a hindrance, but perhaps an arrow grip where the claws are used to hold and notch the arrow, then pull back the string? If need be they could cover their string claws with some sort of padding. Or just a pinch draw.
So there it is. There's more to this species than what I've listed but I'm already worried this is too long. So is the described creature scientifically viable?