Rape time!
This just doesn't fit the theme of Kingdom Hearts. I think Kingdom Hearts is a game more about the what happens to a heart when it leaves the body. Thus their intent was to leave the explanation of what happens to a soul when it leaves the body up to local religion of each individual world.
Eh, wrong. The overall theme of Kingdom Hearts is not just hearts. Ever heard of a little game called CoM... See, now, that entire game has to deal with memories. So... hearts, you say? Their intent was to leave us guessing, and since both the heart, memories, and body have come up, it's only logical to assume that the soul woud, too.
So far all that has been explained is how the heart would separate from the body (being engulfed in darkness), what happens to the heart after leaving the body (becoming a heartless), and what happens the the leftovers, the body and soul (creates a nobody).
Yes, because that's all that was needed to be explained as of yet.
Now a soul leaves the body through destruction of the body. However, what happens if the soul was forcibly removed from the body without going through death? This is where I think unbirths might come from. Rather than unbirths purely being the soul, they are the heart and body, both still in working condition. (for those people who are Harry Potter geeks, think the dementor's kiss)
While the others already said it, this is still rape time, so I'll say it, again, just for good measure.
ONCE THE SOUL LEAVES THE BODY, THE BODY DIES... COMPLETELY.
This would fit Kingdom Hearts as it deals with yet another situation the heart could be placed in. Also would help explain the name unbirth. As a creature that has a heart and body, but no soul, has not been truely born yet.
Eh, nope. See above statement.
the other, and less likely opinion is they are the heart and soul, without the body. But a soul purely on its own would just go to the "afterlife". And that's ground I don't think game developers want to step foot on.
Eh, this makes just as much sense as the rest of it, and you practically say that. The soul, if gone from the body, would leave, and the heart, if gone from the body, would leave, leaving just an empty corpse, without even the chance of becoming a Nobody.
Also, the game developers probably don't give two shits what ground they step on. If it weren't for Disney, they would probably go a lot farther than what they already hint at. What they already do probably steps on some ground for some religions, so that probably does not matter to them.
Whoo, this was fun, although since you were actually intelligent about it, I like the post.
Basically, your entire post crumbles because the soul cannot leave the body and let the body live. The rules here don't follow the rules that Harry Potter does.
See now, no one remembers that I didn't like the idea of Ven's soul being in Sora, either.