I'd exchange the first part of your theory with the theory that MX forcefully removes Terra's heart and inserts his own. That's probably why Aqua and Terra would be fighting.
Other than that, I think that Vanitas also has a connection with Ven, Roxas, Riku, and Sora, but I have no thoughts as to what that connection is. If you ask me, I think Vanitas will look like Sora. I don't know why, but I'm sure Nomura will give a reason for that.