Yeah that makes sense, if Ven's heart was in there for too long it would of became a heartless or just ended up in Kingdom Hearts I didn't think of that. And seeing, that we know almost nothing about the soul, it leaves us more options to make theories about. But how did Roxas end up with a heart, did the soul of Ven call back its heart back?
Wow sounds very logical, exepts for one little thing, it all adds up in my Opinion.
For me, it would make more sense if Ven's Body and Heart were in the corridor of darkness and that Soras Soul had inhabited Ven's Body and Heart, giving Roxas his own heart and aswell as explain why he looks like Ven. That why Kairi could somehow called back Sora's body giving the Heart a vessel.
But if i think about it, Sora would've been dead if that were the case, seeing a body and heart with out a soul is dead in the KH universe... or maybe he got a part of Kairi's Soul or something.