The GAC can help you wait out of the sun wherever possible, though a large number of attractions have indoor or shaded queueing. I'm afraid I don't have any personal experience of using this, but other people here have used it, and it seems to work for them. You should also be able to get a stamp alerting CMs to the fact that you may need to leave quickly. For something like Fantasmic, this may mean allowing you to be seated in the rows reserved for those with mobility issues or travelling with wheelchairs (the row or two in front of the wheelchair seating). Just explain your needs to the CM at Guest Relations and they should sort you out as best they can. If the provisions aren't adequate, you can go back and see what else they can do.
Being in pain can be extremely draining, and walking around in the heat may be too much for her. It might help if you rented a wheelchair or stroller, to help her save her strength for the fun stuff. Disney involves a lot more walking than most people do in their day to day lives, and the heat can add to the stress on the body, so those of us with existing medical considerations sometimes need a little extra help. She wouldn't have to use it all the time if she felt like walking for a while, it would just be good to have, to help her save her strength when she needs to.
If you
rent a stroller, this will provide her wish some shade, but if she is too big for this, or would feel uncomfortable using a stroller (I don't know how old she is), you can buy parasols with clips so they can attach to a wheelchair. I know you can get these outside the Hall of Presidents in MK, but they're probably available elsewhere too (I would guess you could get them in some parts of the World Showcase). A clip on fan on the stroller / wheelchair might help her keep cool too. Oh, and if you go for a stroller, you can get the "stroller as wheelchair" stamp on your GAC, to allow her to rest in it in lines.
If she becomes weak or sick suddenly, you might want to consider renting a wheelchair, stroller or special needs stroller from an external company, so you can take it out of the park with you, and load her onto the bus in it. It would also mean she could use it around the resort, getting to and from buses, and at DTD.
Hope you have a great trip, however you choose to play it, and don't be afraid to ask for whatever help you need!
