I think that Boardwalk Villas might be a great choice, if you can swing that. the advantage of that one is that it has a sofa-bed in the living room where she can sit during her non-park times - very comfy. If you got a studio, it has a fridge and microwave, and if you get a 1 or 2 bedroom, there is a full kitchen. That means you do not have to go to restaurants for every meal, which takes some pressure off. My experience from traveling with elderly folk with mobility issues was that they liked to be able to hang out and relax, and they retire much earlier in the evening.
Your other options are Beach Club, Yacht Club, Swan and Dolphin.
I don't know if she needs constant supervision and you have help, but just in case, you should be aware that Fairy Godmothers does provide sitters for special needs kids and adults who need supervision. They don't do nursing care, but they do provide supervision. Their number is (407) 969-9847 and toll free 877-611-4314
Monorail from Epcot to MK is more of a pain than you may realize, when you have a person in a wheelchair. You have to go up and down a couple of steep ramps twice in each direction, because you have to change monorails. Very tiring. It is up to you...I hear the Grand Floridian is lovely.. but I would choose BWV... or OKW if you are going to use a car.