I'm truly sorry to hear that you have additional complications - and that your GP has recommended "wheels only".
But hey! Just remember this:
If the worst thing that happens on your trip is that you have to use an ECV... you are a pretty lucky kid! 
You'll still be at
DISNEY WORLD, my most excellent favorite place to visit in the world! Lucky puppy you!
I hope you decide to go, because two reasons:
1. It's DISNEY WORLD!!! How can you NOT go, if you have the chance? If I could, I would be there once a week! (It's a bit of a commute from Oklahoma, so...)
and 2. In all seriousness, when I am at Disney World, I forget all about my pain. The endorphins kick in, and I smile and laugh and feel more like myself than anywhere else in the world. The effect is so pronounced that my DH literally is ready to move to Orlando if he can ever get a transfer (Orlando is the number 1 requested transfer within his company LOL)
I want you to know one last thing: Since the first time I had to use an ECV, nearly a decade ago, it's actually gotten SO much easier at WDW. Yes, there are folks who just don't pay attention. And little nuggets who don't understand the laws of physics, and dart out in front of you. But, even if you are walking, those things happen. It's not just ECV drivers that get bumped into, have people dart rightinfront of them, and have to endure the Guests who leave all common sense at the Main Gate.
I hope, whatever you decide to do, that soon you will find yourself on the road to a complete & swift recovery.