There're also the Bella's old room theory, Bella's dead son's room theory, Husband in coma in the room theory. What else did I miss?
Edit: the above is on the room side. On the husband side we have... husband looks like Neil, husband MIA in war, husband ran away (either because Bella changed to being mature and stares too much, or she was too wild, or he cannot cope the grief of the dead son, or Bella cheats on him ONCE with Jill)
Edit edit: Oh, there's another popular about Bella being a dominatrix and that room being a torture chamber.