Glad you found your way over to disABILITIES, which I was going to suggest. We visited last Spring and my Mom rented the ECV every time we went to a park. She was thrilled and wished she had tried one sooner. She was afraid of driving into walls but said the thing was actually very easy to maneuver.
Please make sure your Mom is ready to ask for all the help she may need. My Mom had a hard time walking from the shop where you return the ECV to the handicapped parking spots. We now know that if you just ask, a CM will accompany you so Mom doesn't have to walk at all. Disney will also provide you with a wheelchair to use all week, if you are staying on-site and one is available. I now request this when I make our reservations.
Disney is my Mom's favorite place, and now she realizes that asthma and limited mobility DO NOT need to keep her from enjoying WDW. Hope you guys have a great trip!