I.e., that the person we know as our protagonist is the product of James and Lily's son progressing linearly through eleven years in exactly the way that we've heard from various in-story sources, and not the product of any other, unknown influence whatsoever.

When I think of in-story sources about HJPEV's upbringing, I think

McGonagall took a deep breath. "Harry, I've seen many abused children in my time at Hogwarts, it would break your heart to know how many. And, when you're happy, you don't behave like one of those children, not at all. You smile at strangers, you hug people, I put my hand on your shoulder and you didn't flinch. But sometimes, only sometimes, you say or do something that seems very much like... someone who spent his first eleven years locked in a basement. Not the loving family that I saw." McGonagall tilted her head, her expression growing puzzled again.

So, um. I'd say 'not very well supported at all'.

How well supported at this point is the theory that HJPEV is... unmodified, if you will? I.e., that the person we know as our protagonist is the product of James and Lily's son progressing linearly through eleven years in exactly the way that we've heard from various in-story sources, and not the product of any other, unknown influence whatsoever.

I am almost convinced that there's some other influence at work, but I don't know what to attribute it to. His oddities, especially his "dark side," could be from the Horcrux (assuming he is one in this 'verse) or something else from canon, which Eliezer would have taken to a more logical conclusion than JKR. On the other hand, rational!Riddle (or another unexpectedly rational entity) could well have done something drastically different from our canon expectations, and that could be what's made Harry the rather unusual creature that we know.