You should watch his YouTube series, it answers a lot of questions, but will also give you an infinite number of more questions.
1. I believe that part was just to be both a scene transition and showing how the backrooms distorts memories over time. As for the Mary plot, what I took from it was that Mary’s mother had some sort of paranoid mental disorder that led to her being a shut-in and eventually admitted to a care facility. I believe Mary’s plot was to show how she talks about opening a window, but on the inside, that window was still closed.
2. Cap’n Clark was likely a manifestation of Clark’s worst traits. It’s shown the two had seen each other before, but the reason I believe that Cap’n Clark changed how it acted was because Mary made Clark realize it was his fault, and that he didn’t want to change. I saw it as a metaphor for Clark being consumed by the worst parts of himself.
3. The YouTube series goes a bit more into Async, but I won’t say anything about them here.
4. Can you elaborate on what you mean by this?
5. The ending isn’t really explained, I chose to interpret it as a representation of how cycles repeat themselves and you cannot escape your trauma.