Will she agree to using a wheelchair? Maybe she is afraid of driving an
ECV and doesn't want to admit it. With a wheelchair she can have you push it, stay in it in almost all the lines and wait with you, she can walk so she can get out and ride with you in a regular car, and if she wants to try walking a bit you can leave it in a spot like in fantasy land and walk around that area and come back and get it to go the longer distances.
I would consider the fear of driving one factor seriously. Will she use one at
Walmart or the grocery store? can you have her try one there? If she doesn't like to drive or doesn't feel confident of her abilities I can see where it would be very frightening to drive one at Disney.
The other thing is would she be perfectly happy sitting on a bench and people watching and interacting with her grandchildren? When my Mom got older she completely enjoyed sitting on a bench being the "holder" of things and just watching her grands enjoy them selves. Sounds bad to us but she was very happy.
I would try for the wheelchair though for the distances.