The more I think about it, one of Nomura's most frustrating habits throughout the series is his tendency to introduce questions where the answer seems really obvious to fans who put two and two together based on simple logic, only for it to turn out no it's actually much more, needlessly convoluted than that whether it makes logical sense or not.
Fan: "So this Ven guy, who looks identical to Roxas- his heart has actually been inside Sora's this whole time? Ahhh, so Roxas has actually been Ven inhabiting Sora's nobody, that's the most logical route to go down".
Nomura: "Actually no, Roxas and Ven just look identical because Roxas' appearance was influenced by Ven's heart being there, but they're still totally different characters".
F: "Ooh… but then how did Roxas become his own person?"
N: "Because, you see, nobodies actually regrow their hearts after a while. That's actually the real reason the other Org members can seemingly feel emotions: they aren't remembering how their emotions felt, their hearts grew back".
F: "But then, if Roxas is his own person, shouldn't the Org members nobodies and somebodies now be seperate too? Does Lea have Axel's regrown heart trapped inside hi-"
N: "Shh-shh-shh-shhhhhh… don't think about it".