In the lounge car,there is a table with an open space with no seat meant for wheelchair use. I don't know if there is something temporary to put there if you don't use a wheelchair. The crew probably can't put a non fixed seat in for safety reasons, but if you brought one, I don't know?
A folding chair would likely be refused any number of places, but a rollator is considered a mobility device, and would be permitted onboard and elsewhere. They're not terribly expensive.I don't think they'll allow you to bring a folding chair for that purpose. I'm just going to lose the weight so I can fit in the booths comfortably.