This is such an awesome trip you're planning for her! I have traveled with folks ranging all different ages and health conditions. Much has been covered, but I think the overall major thing is to just commit and accept a slower pace than you would normally do. While you wont be sprinting from one ride to the next to cram the most in, a more leisurely pace is nice b/c you get to take in things you might usually let pass you by.
Pro tip: Mid-day break is essential. I would plan it around lunch time. This gets you out of the park when it is most crowded and hottest! (the heat will probably be more of a factor for Goma than you are used to). This is the perfect time to go back to the resort for a nap or to cool off in the pool and "reset". If she doesnt want to travel all the way back to the resort, you can still take a break elsewhere. The monorail resorts are great for MK breaks. I rarely have an issue getting a same day reservation at The Wave for lunch which is a nice affordable table service. Or this could be when you plan your tea at GF. Even if tea is not in your budget, the GF lobby is incredibly relaxing with comfy furniture and you can listen to the band or pianist playing. I sometimes like to do Capt Cooks for a cheap lunch at Poly and eat the food in the more comfortable lobby, and pick up a dole whip there with a shorter line than MK.
Taking a break from the parks to relax for a bit, even if its not at your own hotel makes the day much more manageable. You can also consider the crescent lake resorts for HS and EPCOT days, and AKL for Animal Kingdom (I think she would be ok in the front for the safari. But if not, AKL does do some tours at their savannah. dont know if that is in your budget, but would get her to see some animals!)
Suggested dos and donts:
MK DO: Tiki Room, Country Bears, Carousel of Progress, Peoplemover, Pirates, Jungle Cruise, Haunted Mansion, small world, Peter Pan, Under the Sea, Mine Train, philharmagic
MK DONT: Space Mountain, Thunder Mountain (Space is much rougher than
Disneyland's version, and Thunder can toss you around. I dont know what health issues she might have, but she may need to "brace" herself on these ones).
EPCOT DO: Spaceship Earth (if it remains open), Living with the Land, Soarin, Figment, Frozen, American Adventure, films for Canada France and China, if budget is a concern you can grab a cheap lunch at Sunshine Seasons which has tons of yummy options, various performers in world showcase, for an affordable sitdown meal I recommend Chefs de France (they have a pre-fixe dinner menu, a rare find at Disney!)
EPCOT DONT: Test Track (the stop and go portions could be a bit too much whiplash for her), Mission Space (I hate this ride b/c it makes me sick, but there are frequently older folks who feel ill after riding it). Also, dont spend too much time in the sun in World Showcase, it can take a while to travel around there and for Goma, be prepared to find some shelter from the rays as soon as you get to each pavilion
HS DO: Runaway Railway, Toy Story Mania, Muppetvision, Tower of Terror (if she's ok with drops), Millennium Falcon (Id put her in the engineer role since its less "video game-y" than the other two), Walt Disney Presents, Littler Mermaid, a fun themed meal at either 50s Prime Time of Sci-Fi Dine In
HS DONT: Rock n Roller coaster (bangs your head around), Beauty and Beast live (I swear that theater is 25 degrees hotter than the rest of the park for some reason), Indiana Jones stunt show (only if its a particularly hot day. This and B&B are fine if the weather is more agreeable)
AK DO: Kilimanjaro Safari, Finding Nemo, Festival of the Lion King, Flight of Passage (you may have to help her on/off the ride vehicle), Navi River Journey, Maharajah Jungle Trek (if its not too hot), Tough to be a Bug
AK DONT: Dinosaur (I find this one REALLY whips you around), this park is also going to be the biggest problem in terms of heat (how did they put SO many trees in a park yet NONE of them shade the pathways!?) so DONT forget to keep hydrated and dont be afraid to step into a store for a moment of AC
AK Maybe: Expedition Everest (for a roller coaster this one is a pretty smooth ride. But it is a roller coaster and therefore fairly intense. Have to gauge what she is up for)
Long post, sorry! I hope you have a great time on this trip!