Although Bella is my least favourite LI, she is the one major mystery in this game...
...My Theory:
I'm going with Bella & James had a child who was tragically killed in an accident. The couple drifted apart emotionally and each found solace in another partner (Bella with Jill, and James with some other woman - someone we may already know
). Both accused the other of damaging their marriage and they separated, with James leaving.
The lower mirror is for the little kid since the normal mirror is too high (I've got to admit, I've never seen this before, most people just put a step next to the sink, so a kid can brush their teeth etc.)
The locked room is the child's room; a room that Bella doesn't want to share with others, and can't bear to clean out.
Bella is asking James (in her mind) to forgive her for her affair with Jill, and she says she forgives him for his affair with the other woman.
Bella runs from the library that time on her date with the mc because there are still strong emotions surrounding her husband, their child, the marriage breakdown, and going there was forcing her to confront some things (this is a fucking stretch, I really don't know why she ran off like a maniac).
It was James that ultimately left Bella, and she never fully accepted the marriage was over, that's why she still wears the ring and wasn't looking for another relationship.
I believe that Jill knows of Bella's history that's why she's defensive when the mc starts poking around, perhaps because she knows she had a hand in it. She probably wasn't aware that the child's room has been locked all this time though (since she's typically not the kind of person to go snooping around until the mc brought it up).