Sounds plausible enough.
Another theory: Since it's called "Dive To the Heart" I wouldn't be surprised if it was the wielder's heart that called them there, and was
talking to them. That would explain how they ended up there, why they could hear a silent voice, etc. Plus, the wielder or soon to be wielder fights their shadow, technically, which could be symbolic of them fighting the darkness in their heart and trying to overcome it to become a wielder.
This would also explain why Roxas and Sora fought at the Station of Awakening.
I wouldn't be surprised if Ven's heart called him there, either.
That's plausible too. The one thing I've always disliked about it, though, is the fact that their heart can be personified into a voice and know that they are being chosen for the keyblade.
Mickey, on the other hand... we know he can act as a disembodied voice (KHFM, when speaking to Riku in the Dark Realm), and we know he can not only talk to Riku remotely, but appear inside his being (Re:CoM).
It just fits perfectly, especially consideirng the whole thing about the Disney keychain insignia implying that he made the KK/IKK.