Hi Lycanthera. What an interesting post you have here, makes me rub my paws together to dissect it a little!
First of all, I noticed your mention of schizophrenia and supposed separation of identity, and wanted to correct some information there. Schizophrenia is often mistaken for dissociative disorders, but in general they are not the same thing. I believe you are referring to Dissociative Identity Disorder (DID) and Other Specified Dissociative Disorder (OSDD, which is more of an umbrella for those who show symptoms but don't directly fit the diagnosis of the known dissociative disorders), which are disorders where one's identity (partially) scatters into several. For DID, symptoms like dissociation (episodes of disconnect from the self) and amnesiac barriers (when the different identities/headmates do not share the same memory) are a key factor of this. From what I have gathered over the years, this disorder is indeed a survival mechanism from early childhood. As a child, your sense of self is inherently more scattered over several types of emotions, which are supposed to merge into one sense of self as your brain develops. But ongoing trauma may make your brain prevent this merge from happening to preserve the ability to function, causing a more defined split in identity.
Knowing this, from your theory it sounds like that'd have to mean that from early childhood, all humans should already have some kind of animalistic side embedded in their brain. You mentioned this as a shared evolutionary animalsitic mindset that all animals share, but that the humans have come to suppress overtime. While it's true that many animalistic instincts of humans have toned down over the years, I think it's important to add nuance to this: evolutionary instincts do not appear out of nowhere, they are a result of longterm conditioning of a species to be able to survive. Therefore, all instincts can be traced back to a cause. The Uncanny Valley theory is not a random instinct, but instead can psychologically be traced back to a combination of the human's detailed perception of the human face (to be able to recognize emotions, and therefore to help with social survival), and the detection of danger when a person's face feels off (which also stems from xenophobia).
The fact that humans experience this only further proves that a set of instincts per species are more species-specific than you'd think,
because no other animals have the ability to read and communicate with faces this well.
As per your theory, it'd leave me to question why some therians would experience prey-like instincts (flight and freeze responses to being attacked for example), while others experience predator instincts. Why some therians have the urge to swim, while others will want to fly. That's no one collective animal mindset, that is a whole respective framework for each individual.
I am a huge fan of dissecting natural human instincts from animalistic therian experiences. I solemnly believe there are overlaps and similarities, but also undeniable distinctions between the two. To pin therianthropic experiences purely on our shared human (or overall animal) instincts personally feels a bit like tarring it all with the same brush, as if all animal instincts come from the same source.
So please feel free to add your thoughts on this. Maybe you do have explanations to the nuance I've given, so I'd love to hear your thoughts on that.
Overall I think it's not an impossible theory, just one that deserves more attention in the detail