Alucard =\= Trevor
Alucard = Trevor + Another Person
If we assume Cox isn't lying to our faces, then that would be a very possible explanation. I wouldn't be surprised if the Wandering Soul is Zobek and ends up possessing Trevor, turning him into Alucard. It would explain that one impression someone wrote about Trevor transforming into a shadowy version of himself, as maybe Alucard is akin to a split personality. And if Zobek can possesses people, it would be one possible reason why he looks different from his original look at the end of LoS.
That being said, I really don't buy the Satan theory.