Maybe Alessa was somehow captured and forced to combine with a Claudia like character, then the Claudia character needs to combine with Heather to birth god, as for why the cult suddenly wants to birth god, they realized that the demon they thought was in Alessa, was really a god, and as for how Harry/Christopher got in Silent Hill, Alessa saw that her other half was unhappy, and let a hole from the real world into the otherworld for christopher to find, but made Rose stay for some reason or another, (If you've seen the Ring 2, like how Sahara wants Rachel to be her mother or something,) then Christopher helps them change their names and lvies, then eventually the cut/claudia-alessa merger tries to find her and all. I just REALLY hope there's a Vincent-ish character, call me crazy, but Nicholas Cage would do a great Vincent,
But then again I doubt they'd choe an unknown for a main char then a mainstream star for a supporting char.