If you have not already read the FAQ sticky at the top of this forum, I suggest it. It was very helpful for when I took my sister back to WDW after she developed stamina and panic issues.
Since you have the wheelchair, unless she has some invisible issues, you should not need a GAC, but I would suggest stopping at GS and asking for maps for those with disabilities. It will help you figure out where you might need to go to enter before you get there. I know everyone says to ask the CM at the entrance, but I have gone to some attractions where for some reason there was not a CM. You can already find these on the WDW main website if you want to get an idea. It will also let you know which attractions she will need to transfer for and which she can stay in her wheelchair.
As for shows, I can never remember where each show has HC seating. I do remember FotLK was the front, Bird show at AK was middle or back, Indy was back with standing room only behind, and Beauty had some in front and back.
For parades, there are handicap locations, but you still want to get a spot at least 30-40 minutes early. For some parades even earlier. Depending on how many there are of you, if you want to use the accessible spots, you might want to plan for 1-2 people staying with your aunt and the others finding spots near. Here is information on the ones I can remember
MK: Across from the entrance to HoP as there is a low wall that non-wheelchair companions can sit. It was great for the Electric parade so that after the parade we were not in the crush of people trying to then move for a better spot for fireworks.
MK: At the flag pole in front of the train station. This was again for the Electric parade. It was good for having a front view as they came out from offstage and made the loop around the flag pole. It is good for getting out of the park before everyone else moves as you are the first area where the parade finishes.
DHS: Going to the right of the tree when coming in from the entrance across from a food/drink stand with the path to Dino land across from it. There is some shade in this area that is good. Although, we once parked here and there was only us and one other handicap party so they opened it up and that made it a bit congested. My sister likes this one so we have not tried the others.