I haven't read through every page of this thread, but I think Kyoto might be worth considering. First because it's very flat (although some streets use brick pavers), second because you can reliably get clear soups, Wonderbread, and scrambled eggs in restaurants, and third because it's staggeringly beautiful. But also because cultural respect for the elderly tends to translate well for people with mobility issues. I saw people using wheelchairs and walking crutches on the bus and train when I was there.