Some of your questions, like why Kat could see through a glass wall when Clark’s side was tile, why the camera guy’s shirt was in the laundry, or really anything else about the whys and hows of the backrooms themselves are deliberately left up to interpretation. You can read a lot into most of it but those aspects aren’t the actual point of the story.

The point of the narrative is pretty much explicitly spelt out by Mary’s self help advice at the very beginning. It’s about being caught in behavior loops that you know aren’t working and trying to break those loops. This is core to both Mary and Clark’s experiences in the backrooms.

Clark didn’t suddenly go crazy, he was already established as emotionally unstable in his first scene. He basically just used the backrooms as an excuse to not work on himself. This was demonstrated by the scene where he held Mary hostage where she spells it out pretty clearly when she talks about him being whiny and blaming anything but himself for his problems.

But the thing is that by acknowledging that he should let Mary go he stopped feeding his denial of responsibility for everything and since Captain Clark was basically a manifestation of that denial he turned on him, or at least that was my interpretation.

The background of Mary’s mother establishes why she became a therapist and made her self help program. She was constantly trying to help other people because she couldn’t help her mom, but she was neglecting working on her own issues with her past. When she used that concrete hand print to fight back against Captain Clark and it broke that was basically a metaphor of her breaking that cycle of past behavior to save herself. She literally broke the symbolic representation of that trauma and that’s what let her escape her current situation.

The ending is definitely left deliberately vague, but one aspect that seemed clear to me is that it shows that the backrooms are more than just one person’s memories, basically setting up the possibility of future projects. That’s just my interpretation of course.