@RandomBuzziness If the sims have a high enough friendship they will use the bathroom and not be embarrassed or shoo, last I knew. I wouldn't worry about that. Same goes for sharing a double bed.
Sounds like the roommate is doing fantastic! I expect there are are a number of factors that can effect various aspects of the roommate situation. And in my experience of Sims there seems to be a bit of random chaos factored into the algorithm with certain traits and situations. I'm a micromanager in my game, so having sims I cannot control in my home is a bit unnerving - and a great way to mix things up. Anyway, whatever the known and unknown factors are, your roommate and sim seem to be a great match!
As far as the non-answered ad, you could try taking it down, leaving the lot/game, re-entering the lot maybe make sure there is an extra bed plus all of those items you mentioned that you have for the current roommate (the dresser, bedside table and desk). Then put the ad back up again. I'm playing a different save now and am finding running the ad isn't working so great when I leave the lot with one of the sims. Also, it can take 1-3 days for them to start to come, then they taper off more if I don't pick one for a while, it seems. Anyway, a few things you can try.
Looking forward to hearing more! The information your shared is very helpful and well tested. Thanks!