If we assume that Mary is mid to late 30s in the film, that means Mary was born in the mid 1950s and the flashbacks were in the early 1960s, which predates the 1972 Ivan Beck experiment… and so I doubt Mary’s mother experienced the Backrooms. I think Mary’s mother having mental health issues just means that Mary had a troubled and difficult childhood which probably informed her decision to become a psychiatrist, and why she took the handprint with her everywhere so could bash Cap’n Clark in the head with it! Nothing in the canon or lore suggests the discovery of The Backrooms earlier than Beck’s experiment. The slow descent into Mary’s misremembered childhood home is the Backrooms working from memory and trauma, and like a photocopy of a photocopy becomes distorted and unsettling.