I think you should be prepared to help her with the
scooter when necessary. Loading a scooter on and off the bus or whatever is not easy and it may be better if someone else in your party did it for her. Practicing at the local store won't prepare anyone for this situation. You will probably have to give her a boost on and off the bus, as well. There are some rides that are awkward with the scooter like Soarin' in Epcot. You actually have to do the scooter on an elevator to get to the level of the ride.
Another hard time with the scooter is getting it in and out of the resort room. Again, it may be better for someone else to do this for her.
If you can go to a local zoo for a day with grandma included then you can rent a scooter there and get in a good practice day before going to Disney. You can see for yourself that this will be doable or not at Disney for her. The local zoo will still be easier than doing Disney.
The park challenge with a scooter is how invisible the scooter is to everyone else. They will step out in front of the scooter with no consideration to the scooter driver. You can safe guard somewhat against this by surrounding your grandmother on all sides. If you have enough in your party it would be great to have someone in front of the scooter leading the group and people on her sides and one in back. You become her eyes and you can give verbal commands to stop or whatever if necessary.
We used all these techniques when we took my MIL and she was 87 at the time. One funny story I will share, here. My MIL seemed to want to rub her nose a lot. Well, every time she did it she would let go of the scooter handle and the scooter would stop. She never did catch on to this problem; so, we would pay attention and tell her to keep her hand on the throttle at all times when moving.
You need to shut the scooter off every time you are at a show, parade, or fireworks. You want to conserve the battery power especially if you plan on longer days in the park. Keep the battery charger with the scooter; there's a pouch for it. If you run out of charge there are places in the park where you can charge it if necessary. This will take a while if it becomes necessary. The scooter has a charge indicator. Make sure you are checking it every hour or so; so, you know it still has enough power left. There is a manual lever usually in the back and towards the bottom of the scooter where you can put it in manual mode which is easier to push if necessary.
Each night you need to charge the battery all night; so, the scooter has power to run the next day. Don't take this for granted and don't depend on your grandma to remember or know how to do this.
If this all becomes overwhelming to her you may have to consider going to a manual wheelchair and pushing her.