We've been there 3 times at the end of September, and it's always over 90, so AM touring is a great idea. Also, be prepared for quick rain showers that often don't last long. That time of year is still very important to drink plenty of fluids, plan on rest breaks in air conditioned buildings, and go slow.
My Mom is your Mom's age, and I'm 57 ...... it would be very difficult for me to push her around in hot weather (and she weights ~ 110 lbs), but I really don't like the heat. It can be a walk from the resort room to the bus or boat and is certainly a walk from the busses to the front entrance of most of the parks. She could easily be tired before she's even gotten a chance to enjoy the parks. Epcot and MGM seem flatter to me, but AK and MK definitely have some areas with inclines.
If there are several of you taking turns pushing the w/c, it would be easier. You can always try it and then rent a
scooter. Since she's mobile, you could drive the scooter on and off and in and out of tighter spaces and let her navigate when it's straight and easier.
There are many, many people enjoying Disney every day with more limited mobility than your mother. There's plenty of sit-down attractions that will provide a nice break with air conditioning, and CMs are very accomodating. Hope you have a very special trip!