I have been riding Superliner sleepers since they were introduced around 1980, and I ALWAYS have a bigger bag I leave downstairs, and a smaller with me. My strategy is once a day when I shower I pull new clothes out of the downstairs suitcase, and shove the dirty clothes back. I never bring the bag upstairs (the showers are downstairs). If I can't handle the suitcase in the rack without blocking the aisle or because the rack is really full, I just haul it a couple of feet to the vestibule area where there is lots of floor space and open it there. Time it so you are not near a stop if you are going to do this.

The rack is right at the foot of the stairs going up to the second level.

There is no place to put a big bag inside a roomette. I am leary about putting a bag under the seat and I don't do that, as there is the mechanism to lower the seat into a bed. I'd only do that if I knew the bag had lots of clearance (like 2 or 3 inches). My "upstairs" bag is usually a backpack that I hang from one of the hooks in thr room, out of the way.

You can use any restroom. Personally, when it is open, I only use the upstairs for a quick #1 . For #2 I always go downstairs. I think it is more courteous, but that is just me, and the restrooms are there to be used.