In addition to the boat option to visit Downtown Disney from Port Orleans French Quarter, there are also buses. Thinking of boats, I did want to point out some rides that are "boat like:" Small World, Splash Mountain,The Jungle Cruise, Living with the Land, Gran Fiesta Tour, Maelstrom, and Kali River Rapids.
When you make your resort reservation, I'd make sure to include a notation about your grandmother's condition and the need for a close building and first floor. Consider requesting building 2, 3, 4 or 5.
I think I'd encourage you to consider table service restaurants & counter service restaurants rather than buffets. I know from our experience that that is more challenging with our mother because someone is having to not only get their own food but also hers as well. When making advanced dining reservations. I'd include a notation that you have a guest in a wheelchair.
If you will be traveling during the heat of the summer, consider bringing an large umbrella so she can use it when she is having to wait in the sun. It will also work miracles when caught in an unexpected rain shower. A large rain poncho is also wonderful since it can cover the entire chair. And don't forget sunscreen on her legs and tops of feet if they are exposed; we learned that the hard way once.
Disney is truly miracle workers when it comes to helping guests with wheelchairs. All of the cast members are so well-trained and, most importantly, incredibly patient and kind when helping guests with mobility issues. You won't have to do a thing! They will let you know where you need to go and help in any way they can whether it is loading the wheelchair onto a bus or knowing where to access an attraction. Your park guides will also provide you with a notation indicating which rides can be ridden in a wheelchair and which ones requiring exiting the wheelchair so you won't be caught by surprise.