I have MS and use a power wheelchair. I can stand up and walk for short distances but can be a bit wobbly on my feet. I go to WDW on a fairly regular basis. There are only two rides in the Magic Kingdom that have moving sidewalk for boarding that cannot be stopped. They are the Tomorrowland Transit Authority (People Mover) and Peter Pan. The other rides that have a moving sidewalk for boarding are Buzz Lightyear Ranger Spin, Haunted Mansion and Little Mermaid.
For Haunted Mansion, if they ask you if she is alright walking a short distance and standing in a short line, say NO. If you say yes, they will direct you to go to the exit, park the chair and you walk "upstream" against the exiting crowd and it is a long walk and you will have to be quite insistent to get them to slow or stop the moving sidewalk. If you say NO, they lead you to a different area to wait in your chair off to the side, when the more accessible doom buggy comes by, they will stop the ride, let you get the chair up close to it and do the transfer.
Both Buzz and Little Mermaid will see the chair and direct you to the exit to board and you just tell the person who comes to help you board that you need the belt slowed or stopped. Both of those rides also have a car where she can stay in she chair and be tied down into the car.
Winnie the Pooh does not have a moving belt but does have a wheelchair car if she needs to remain in her chair.
There are three rides where your ride vehicle is actually a boat, Pirates of the Caribbean, Jungle Cruise and Small World. The step down into the boat and back out again are difficult for me. As a result, I no longer ride Pirates. However, both Jungle Cruise and Small World have a boat that they put the wheelchair on and you stay in the chair. For Small World you go through the gate marked with the handicapped sign and down a ramp. At the end of the ramp a cast member will ask you if she can transfer, if you say no, they put you into a separate line to wait for the accessible boat. For Jungle Cruise, if you have a fastpass, a cast member will direct where to go to wait for the accessible boat and if you do not have a fast pass, they will give you a card with a time to return and when you return put you into the accessible boat line. However, if your Mom can do the step in and out of the boat with help, tell them and you will not have to wait for the accessible boat but they will have the boat tied down and held steady while you help her get on and later get off.
Many people with MS have problems with over heating. If she has heat issues, the best solution is wearing a good hat, having a little personal fan, a cooling towel (google FrogTog) and drinking lots of and lots of water. Every quick service restaurant will give you free cups of ice water, you don't even have to order food, just ask for a glass of ice water.
