This may or not be helpful, but I'll put it out there.
I've had trouble with my feet at WDW for many years. Over the past few trips, it had also involved my ankles, hips, and lower back.
Turns out, everything stemmed from plantars fascitis -- its the strip of tendon or muscle between your heal and the foot pad below your toes.
I had a lot of tightness that went from my toes to my glutious maximus. I did a ton of exercises, got comfortable shoes, inserts and HEEL CUPS, oh and ibuprofin and had the best trip EVER. My back never hurt at all.
My feet only hurt twice during the whole trip. Once after a double whammy long wait for both muppets and then little Mermaid, and the 2nd time at security at the Orlando Airport. Both times, I had skipped the ibuprofin.
That surely beat being miserable at 11am every day, knowing all of the best bench locations throughout the trip, nearly passing out waiting for busses after fireworks, and being totally useless once we got back to the room.
Here is what didn't work. I've put 2 tubes of aspercreme on my feet in 1 week, patches on my back, elevating my feet while I slept at night, and ate bananas.
My sister has scoliosis and know that back pain can be horrible. I hope this helps.