Hello Rednaxela,
Depending on what you want to do, queues could be an issue. However, if you steer clear of peak/rush hours you may not have many lines at all to deal with. I avoided these times myself and I didn’t have to wait long in any place I visited in Tokyo, Kyoto, Kobe, and a few other cities.
As for using a folding chair in long lines, you may get some looks but I don’t think it will be rude unless you block foot traffic for others trying to go around or something (if you’re using a walker with a seat, then you probably wouldn’t get a second look beyond curious onlookers). Finding a place to rest will also depend on the individual location, but because of Japan’s aging population it becoming more popular to have resting area throughout many places. Also, on https://www.accessible-japan.com/ (check it out if you haven’t already), the Accessibility section of many of the attractions reviewed often note if there are resting areas available, great for some pre-planning! Generally speaking, I think you’ll find a seat when you need one. 🙂
If you have any other questions, please feel free to ask and we’ll try to get you an answer!
Hope that helps,
-Justin