It seems clear that the relationship between Trevor and Alucard is the last scene of the game and connect directly with CGI of LOS2 we saw.
It is possible that like Gabriel/Dracula, Alucard is the evil part of Trevor, remember CGI of LOS2 when Alucard seemed have a mask on (and therefore saw his reflection in the sword), but that does not mean that the good part of Trevor ascended into heaven, may have been misled by Zobek and wear the "mask of evil" evil hand separates.
Anyway something does not fit me .. If Alucard born from Trevor (both live or die Trevor) spend 30 years until Simon comes. If Alucard wants to kill Gabriel why wait so long? .... According to Cox all LOS2 CGI scenes pass after MoF, so that encounter between Alucard and Gabriel is after Simon...