I wonder if what they really did with her mind is fix it to a certain reality. As in, the random people in the town don't notice it changing, Crest and Regina notice because stuff happened and their minds are able to notice reality when it changes phase. Suspiria is able to see one reality, and if it changes, she doesn't.
So in the case of Suspiria's parents dying:
1. The townspeople would believe whatever their current reality said to believe, instantly overwriting when stuff changes. In the reality where the parents died, they might feel bad, otherwise they think she is crazy.
2. Crest and Gang notice stuff like outfits and architecture and the number of deaths changing, but also see distinct realities. That is, Crest would believe Crest's parents are still alive because he saw them, but also would be able to observe a reality where they weren't alive.
3. Suspiria, no matter what reality she lives in, has memories from one reality (we'll call it Reality 3a). In Reality 3a, she knows her parents died, and evidence to the contrary in this reality she decides is nonsense.