>>807 Interesting and new theory! I've always imagined Madotsuki to be a lucid dreamer, not somebody who could transport herself to other dimensions.
Theory: The only reason Madotsuki doesn't have a bathroom in her apartment is that Kikiyama was too lazy to make another room and going to the bathroom is not essential to gameplay.
Theory #2: She lives in a place where all the inhabitants have to share a bathroom and the bathroom is outside her apartment. Yes, I have been to a place like this while I was visiting my relatives.
Theory #3: She lives in an attic or basement of a store. The bathroom is located in the store. A part of the reason she refuses to leave her room is that she doesn't have to go to the bathroom/is not hungry.