This is using solely in game sources, no supplementary material will be used.
The center of my argument is the possibility that there are cult members who know what really happened in Silent Hill 1, that are never seen.
In this scene, you only see Dahlia watching Travis about ready to go in there and save Alessa:
You'll notice no one else is seen with Dahlia.... But the memo in 3 in the hospital (The memo is titled Book of Lost Memories and is found where the candle altar is.) talks about this ritual where the victim is burned alive and that only the clergy are allowed to participate, sinners and criminals are banned.
BUT WAIT!
Travis confronts Dahlia and tells her that "they" left her to burn! Dahlia retorts "So we did." She goes onto say "Alessa is with those who care about her."
I know what you're thinking... "But Xuchilbara that doesn't prove anything!", it has a point. The point is none of these other people Dahlia and Travis are talking about are seen, at all. This is important. Especially later on. If you don't consider Origins canon, it actually doesn't matter.
Let's fast forwards seven years...
The unknown two people here knows what is going on, this confirms the memo in 4 asking if they found Alessa yet.
This all happens before 1, but... The only people who know what happened and survived are Harry and technically Heather. Kaufmann is gone, Cybil is dead, Dahlia is dead, Cheryl merged back with Alessa, right?
No, there had to be other people.
Fast forward to SH3, 17 whole years later....
Vincent fishes for information about what happened 17 years ago in Silent Hill 1. In this tape recorded conversation with a cult member she talks about a "group of pagans" and Vincent seems to be disappointed in her story. Why would that be? Does he have information about what really happened?
Vincent has an idea when the events of 3 unfold that he knows what is really going on. He is confused about how Heather doesn't know what is going on. Though the tape recording suggests the seal of Metatron was being used, no one could know that unless they were there. Specifically, that Alessa used it against God.
It doesn't matter what ending you get Dahlia will say she was shocked the Metatron seal was being used:
This means it was a recent realization and you only see this being used correctly in Silent Hill 1. It was just left unfinished.
You play SH3 and Vincent knows to use it against the "false" God and tells Heather to use it, although he doesn't completely understand how to use it himself.
If that isn't proof that somehow Vincent gathered enough evidence to piece together what really happened off camera, perhaps this picture will:

Notice in the back ground it depicts the other world of Silent Hill 1. Whomever carried on this legacy and painted this picture, or had it painted, knew that the events of the ending happened. The baby and the otherworld depict the events of 1. There's also proof in the fact the cult knows that Alessa reincarnated that, this must mean someone saw these events off camera or more than one person. There was no way they could have known other wise. Specifically, that Alessa reincarnated and gave the baby to Harry, which at this point they knew him lest he would not be in hiding with Heather. Even his name and where to find him. This only makes sense if there were other survivors.

