As far as I know, this theory of yours(You need to come up with a better name for it though lol) does not contradict my Original Kingdom Hearts theory. In fact, if we were to combine both of our theories together, I'm sure it would open new pathways to even more answers and theories, based on the most elite knowledge of the Kingdom Hearts series.
Yes, they can work together, but they're two different theories. As I said in the op, I'm not concerned with why the Unversed disappear so much as I am with what they are. They could disappear for any number of reasons which don't necessarily have to connect to what they are (such as your idea of KH splitting).
If you or anyone else wants to use the ideas presented in this thread to further your own thoughts or theories, though, by all means go ahead.
I mean Namine doesn't need to have Sora's body and soul to be made from it. Did you already say something about that? I'm sorry if you did in your op, it's just...so long.
Well this is where it gets technical.
A Nobody is not literally comprised of the same vessel as their original.
We cannot say, for example, that Axel = Lea's body and soul (though most say that for convenience's sake).
Think of it like a chemical equation.
Sodium Chloride, for example, is a by-product in some chemical reactions (like the by-product of a left over body and soul becoming a Nobody).
But you don't say that Na + Cl = NaCl in the sense that (Na +Cl) and (NaCl) are the same thing. (Na + Cl) becomes (NaCl), it's a one way equation. NaCl, as a new chemical compound, has entirely different properties than a homogeneous mixture of Na and Cl.
Similarly, the leftover body and soul become a Nobody. We know that this is true because a body and soul in themselves are considered existent, yet as a Nobody they are non-existent. Plus, we can see they're not the same thing from the physical alterations the Nobody has when compared to the original body/soul.
So Namine was born as a Nobody because Sora's body and soul went into the process as the components needed, but you can't simply say Namine=Sora's body and soul.
She, like any Nobody, was born via the body and soul.
And in that respect I guess you could still argue that the reason why she ages is because her original self is still around.
I thought it was confirmed to be an acronym for 'Darkness in Zero'.
Cause, y'know. He was banished to a world of nothing, Zero, and was left with Hate and Anger, and Revenge. Darkness. Dark memories? It could actually work for your theory. Kind of.
It was. The origin of "DiZ" was to fit a phonetic "Disney" reference in as a name, and it came through the whole "Darkness in Zero" thing.