My theory on how the Organization acquired its hearts for KH;
I think that they took the bodies of those who became Heartless, and forced them to undergo the transformation to be a Nobody. They became the lesser Nobodies. So, when the heart tried to return to its body, it isn't there and the hearts go into stasis.
This explains why lesser Nobodies seem tailored to the different Org. members.
The Organization couldn't access Kingdom Hearts without the Princesses. So they just decided to build their own. The way I see it, whichever heart is the biggest at any given time becomes KH. The KH we saw in the first game was sealed away; that's why they just didn't use it after Xehanort gathered the Princesses.
You have a valid point on Xehanort being wrong about things, but we do actually see in KHII that KH is where the hearts end.
Although the worlds that made the door didn't have to be in KH, the door was the door to darkness, an object with a strong connection to KH, implying that KH was involved in it appearing.
When the door appears in KHII, Mickey says that "The worlds" opened it for them. So, KH contained world hearts.
KH of man/world theories FAIL
Kingdom Hearts is the heart of all worlds. It is where all hearts are born and also end. (Which also disproves KH of man/world theory) It is the great heart, I believe Maleficent calls it.
The door to darkness is the equivalence to a keyhole for Kingdom Hearts. All the world keyholes don't lead to the heart of that world, they unlock/lock the door that does. As evident in Another Report, the Ansem reports, and the door at Destiny Islands.
Xehanort regained his memories as Xemnas. And yet, he still is in half Xehanort/BM half ES form? The only other time we have seen anybody possessed, the result was not a blending, but a straight transition. Now, give me one good reason why Xehanort/BM would take ES's body, and instead of having his own form he takes a blending of himself and ES? It makes absolutely no sense! It isn't because he can't remember his true form; Xehanort abandons his body, and this body doesn't change back to its original form, which for all intents and purposes it should. Without BM's heart, ES's body should go back to normal. Also consider that when this body was abandoned, the heart (Which should be BM's and therefore have his identity) continues to use the physical appearance of Xehanort. (After it takes control of Riku)
Not to mention Xehanort as we know him doesn't look too much like ES. Look at the height comparison; there is a huge difference between them.
Riku exists as just a heart after Xehanort steals his body. He appears much like a phantom in the realm of darkness and is led to the door by Mickey's voice.
Ok, it seems to make relatively believable sense; BM is old and fragile and he needs a new body, so he jacks ES's....except, that sounds really familar....oh yeah, that's the plot of KHI!
But in KHI, it works differently than the theory being presented. When Xehanort takes control of Riku, he doesn't assume the form of a Riku/Xehanort hybrid, he takes the form of Xehanort. And he only shows the ability to assume the forms of Riku or Xehanort. Same with Riku; he can only use his natural form, and that of Xehanort. Considereing Xehanort is no longer in Riku's body, it can be assumed that Riku is taking on Xehanort's true form.
Sooooooo, since Riku and Xehanort don't blend physical appearance, we can say that if BM was Xehanort, he could not blend appearances with ES. Xehanort is, thus, not BM merged with ES.
