You seem to forget the most glaring difference between Ven's and Kairi's heart:
Kairi's heart was healthy and perfectly functionable while Ven's heart isn't.
I didn't forget this, but I didn't think it held any verifiable relevance.
Ven's heart is badly mauled and injured, therefore it can't be released.
How does one follow from the other? The fact that Ven's heart was injured would make it less likely to cope on its own, meaning it would naturally followed Sora's heart (which is essentially its life support), meaning it would have been released.
Furthermore, Ven's heart and Sora's aren't connected the same way as as Sora's and Kairi's, the connection between Ven and Sora is way deeper than with Kairi.
More evidence that it would have been released along with Sora's. However, there isn't any definitive evidence to indicate that Ven's connection with Sora is deeper than Kairi's. Ven's Heart is simply weaker and, presumably, more reliant upon Sora's, but his Heart and Kairi's would have been sharing the same space. As characters, Kairi has a much stronger relationship with Sora than Ven does, so I can't imagine her Heart would have any less affinity towards Sora's; indeed, that's the very reason her Heart fled to his.
On the note of Sora's temporary "vessel" created by Kairi's princess powers I don't really see where the problem is.
I can't speak for anyone else, but my confusion was primarily with the concept of a non-corporeal physical temporary vessel that came out of nowhere. This has since been remedied through various explanations which make a good deal of sense, even if they're all essentially speculative.
Roxas had Sora's body and soul with Ven's heart stuck inside it because the injured heart couldn't remain inside Sora's when it was overtaken by darkness.
Accepting the premise that Roxas has Ven's Heart: if it's so weak, how did it survive while Sora's Heart was plunged into darkness? Ven's Heart is directly connected with Sora's and depends upon it for its continued existence. Once Sora's light blinked out, from what I can understand, Ven's Heart would have collapsed.
Sora was very similar to Ansem SoD, who also managed to retain his human feelings and his own consciousness/mind.
In the first Ansem Reports it it even written by Xehanort that he "transcended" into an existence of only the heart.
Except, Sora wasn't anything like Ansem SoD. Ansem SoD sustained his human form as a Heartless by way of his immense darkness. Sora didn't have enough darkness to create anything more than a mere Shadow. I can probably accept that Kairi's immense "light" could have born a new Body for him, but I don't think it was Sora's powers that gave him form. And, FYI, an existence of only the heart = a Heartless.
The difference was that he didn't have his heart purified of the darkness like Kairi did with Sora => ergo the brown cloak guy we see in KH1 first.
I already addressed this, but it was
because he was so consumed by darkness (and had such a mastery of it) that he was able to maintain a human form. Stronger light gives way to a weaker Heartless. Therefore, there can't really be any relation between Ansem SoD's existence and Sora's. They're fundamentally at opposition.
Heartless and Unversed also "exist" without souls and yet they still are able to be hit and also to dish out stuff.
Heartless are just darkness that has formed from a Heart. Darkness itself has physical elements to it, but Heartless do not have a true Body (or Soul; they function purely on the nature of darkness, which is to spread and consume). Ansem SoD is the exception, but it's possible Master Xehanort's control of darkness was lent to him, essentially allowing him to manipulate his own darkness to the extent that he was essentially conscious. Unversed were physical manifestations of Vanitas's negativity. I don't really understand what element they stem from, but I think it's suggested that they're crafted from darkness given the imagery associated with them as well as the fact that Vanitas's Heart is pure darkness.
Furthermore, about the soul stuff, Sora and Roxas were always closely connected with each other through a psychic link. Roxas even stored his own memories in Sora's heart through that connection (example: Twilight Town in Chain of Memories).
So, the physical location of the soul (meaning in Sora's body which is inhabited by Roxas) is irrelevant since Sora can still either function just the same as a heartless or Unversed (only with his own mind and sense of self) OR he can access his soul through the link with Roxas without Roxas noticing...just like Roxas most likely used Ven's crippled heart to feel real emotions.
I'll agree on this point. Roxas is, after all, Sora, so I can see them sharing a Soul remotely between the two of them regardless of its physical and temporal location, as has already been discussed.
Spoiler
