There's 4 interactable rooms downstairs, one locked in between the toilet and Naomi's room. Truthfully, there's even a 5th on the opposite wall, but you can't even click on it yet.
Nahh, there's Yona's room, Naomi's room, toilet, bathroom, stairs, closet, in that order. At the beginning of the game you get a look into the toilet (I happened to replay the game not too long ago), and it's just a 2x1 room with a toilet and sink, IIRC.
As for the one opposite the toilet/Naomi's room, I think that's a window, unless you're making a joke about the front door.