Not a foot expert -- but it seems if you only have issues at WDW, it's not likely plantar fasciitis (?). Several years ago, when I was trying to cope with PF, the PA at my Dr.'s office said that some of the nursing staff had had good results/relief wearing Nike Shox. I was reluctant for cost and "political" reasons to buy any Nike shoes, but I was desperate and eventually bought a pair. They did not immediately feel comfortable, but after about 6 weeks (longer than you have before your trip), I felt so much better. I haven't worn any other shoes (except slippers, and a few times dressy flats) since -- my heels and back are wonderfully better. I certainly still get tired, achy feet and other body parts from a day at WDW, but not the outright pain of PF.
Different solutions work for different people, I'm sure, but Shox have worked for me for years (even slightly different models of Shox). You might want to use "bodyglide", moleskin, have blister bandages handy, foot soaking, do strecthing exercises, etc. (I used to use a rolled-up t-shirt to pull my foot back toward my shin to stretch the heel in the opposite direction it usually had to be in all day -- that helped some).
Of course, I guess I'm on my feet less now that I'm retired, which also helps (but when I am on my feet a lot, traveling, or doing volunteer work, I haven't had what is called plantar fasciitis since switching to the Shox -- just ordinary tired feet sometimes). Sorry for rambling -- good luck with finding a solution for your situation.