Hm. This is fascinating. Largely because I can relate, and it makes me wonder how much of it is just normal human behavior taken up a notch with you, which was then amplified in your mind and pathologized. Let me preface this by saying that I don't mean offense in any way -- any slights on the validity of your feelings are unintended.
"I've *always* had an insanely malleable sense of self"
"I have a sort of consistent feeling of very minor ego death- in other words the barrier between what I consider 'me' and the outside world is pretty thin"
"I've always been insanely good at manipulating my sense of self, or imaging my sense of self as something different- when I was a kid I'd imagine how I could fight better if I decided to change my sense of self into that of completely random animals at will, for example."
All of these things, I can relate to. Actually, I also happen to use they/them pronouns and feel agender/gendervoid, though I use the label non-binary. It really does feel like the only difference between you and me is that I always dismissed this as normal, and you seem to be more inclined to analyze and pathologize it. It's almost funny reading your post -- all my experiences written out in front of me, but from the perspective of someone who views and interprets these feelings quite differently. If you wish, I can go into more detail about exactly how I relate -- from feelings (and sometimes beyond) that I could create a system if I wanted, to a kind of constant state of ego death, to manipulating my personality at will as a child, but I think you understand what I'm saying. It could be that these feelings are really quite abnormal, I'm experiencing them at a much lesser degree, or even that it's normal for people with childhood trauma, but rarer for the untraumatized. I don't know, but I felt that it was important to point out my perspective on the matter. Have a good day!