I know exactly how you feel OP! On one of my trips I was in so much pain by about half-way through my trip that I didn't know what to do. One late night (after a
MNSSHP much less) I didn't get a seat on the bus. I was practically crying and mentally willing a younger woman I was standing next to to give me her seat. She finally did about half way to the hotel. I think maybe I might have let out a few wimpers from the pain! I ended up sleeping with my feet elevated that night and coated down in Blue Stuff (a topical pain reliever) and more than the recommended dose of Aleve morning and night.
Anyway, my last trip was soooo much better. I had finally found several pairs of good, comfortable shoes! One pair were the Skechers Go Walks that several people have mentioned. Love those. At home I wear them barefoot, but at Disney I wore some low cut moisture wicking socks. I agree they are not the most fashionable, but boy are they comfy! I also found some Teva sandals and a cheaper knock-off version of the Teva's from Kohl's. Both were very comfortable and much cuter, but great support.
As for the heat, I become miserable when the temps get much more than the upper 80s and any humidity at all. My trip in May was warm but the humidity hadn't set in yet. One of the little mister fans helped alot, but I think I will stick with fall and holiday trips from now on. But even at Halloween it can be 85+.