okay! this ranks up in a top percentile for odd questions, but i think the answer is straight forward.
the two conditions added
- devoted to wings that make it capable of flight.
- majority of its mass beneath the skin level
make it difficult to propose anything workable.
an insect the size of a pigeon is not going to be able to fly. The strength to weight ratio is against it. Insect muscles operating inside an exoskeleton cannot exert the kind of power needed to operate wings large enough to lift 1-2 kilos. Birds wings have all kinds of design features that give them aerodynamic advantages over what are possible for insectile wings. So even using 4 wings physics and bio-mechanics are against it. And a fundamental of insect wings is that they are fragile, rigid, and cannot fold.
They have evolved to sapience but are possibly not tool using as their modus operandi is to take over creatures that can do all that nasty manual labour for them. This means the 12 limbs need not have manipulators .. except for whatever is needed to get food to the food inlet/orifice (mouth). Lets say 2 for that leaving 10 for walking.
Now the use of the word "Phasing" to describe how the parasite enters the brain dodges a whole lot of problems that would make it hard to explain how a mechanical connection would work. So the beastie has some way of partly dematerialising so that it physically overlaps the volume of the target creatures brain.
the question specifically said "phasing" which in modern scifi terms means two objects sharing the same physical volume in some manner.
Any method of mechanical attachment requires that we equip the insect with something that would allow it to enter the body of a target organism without killing it. But this raises a host of problems, not least of which is what to do with the wings. Additional problems include how to keep the subject from bleeding out from a pigeon sized wound, how it would connect to the nervous system, and how it would extract itself when the subject host finally expired.
So whatever control over matter-space-time they have for the phasing effect could obviously have the side effects of allowing chemical and /or electrical bi-directional transfer of information .. thus permitting mind control at both of the levels required. And clearly the nuclear option is just injecting a lethal level of energy that fries the neurons.
so .. basically it is a 10 legged insect that could have pincers or claws on the end of each leg according to the authors desire.
A comment from the questioner suggests that he wants an answer that does not break physics, so no "phasing" solutions need apply - in combination with the second new condition any solution is .. doubtful.
A pigeon sized, winged insect might be fit into a human skull, if the brains, and its wings were removed.
For a purely electro-mechanical connection to give the required control over memories the invading insect would have to have direct access to the brain. It cannot co-locate with the brain inside the skull, and below the neck it could only attach to sensory and motor neurons.
So what is left is to make connections through the skull from outside, via its mouth, or other orifice. A connection through its exoskeleton is not possible.
To be able to do this it would need to somehow stupefy or paralyse its intended host, attach itself to the outside of the skull and then burrow in by drilling, or dissolving, though the bone. Lastly it would have to insert fibers or threads capable of carrying neural signals en masse, and then somehow map the hosts memories in the uncharted expanse of the hosts mind.
But an outside connection proposal leaves a large insect clinging to the outside of the skull, which breaks the last condition so .. my answer remains - a ten legged bug that uses phasing to occupy the space between a hosts ears and some kind of "magical" connection to its brain to allow it to read and write the hosts memories.