You may be better off renting an ECV, as pushing a wheelchair can get very tiresome, especially if you are not used to it. Also it depends on the weight of the passenger and the condition of the pusher(s). Some information for off-site rentals are:
Here are the urls and phone numbers:
Walker Mobility:
888-726-6837
www.walkermobility.com
Care Medical:
http://www.caremedicalequipment.com/
407-856-2273, Toll Free U.S and Canada (800) 741-2282
Randy'S Mobility is in Kissimmee
http://randysmobility.com
407-892-4777
Colonial Medical
http://www.colonialmed.com/about_cms.html
800-747-0246
ScootAround
http://www.scootaround.com
Toll-free, US and Canada 888-441-7575
I have used Walker a few times. They will ask for a credit card and charge it the day before arrival. They will deliver the ECV to your hotel bell services before your arrival and pick it up there after your departure. Their rates, especially for longer than a week, are much better than Disney's. Also, their scooters are much better.
In addition, your MIL will not feel that she is being a burdon to others, as it will give her independent mobility. There is quite a different mental state about being pushed in a wc and driving an ECV.
Note that most large grocery stores (and all Walmarts) have
ECVs available for customer use. She might practice on one of these. Be sure to make her aware that the rentals are (1) not as large as the store ones, (2) much more maneuverable as they have only three wheels and not the giant basket, and (3) can go much faster than the ones in the store.