What is the best answer probably depends on how well she can get around. A lot depends on whether or not she can walk/hobble from where she is to where wheelchairs are rented. If she can, you might want to think of that option. If she can't or it is too painful, then use the option of having a wheelchair (IMHO, I would get a wheelchair or
ECV either brought with you or at your resort with the description of her injuries). I think that in most cases, insurance wouldn't pay to rent the wheelchair anyway, (since she can get by without it if she was home) so you would not need to get authorization if that's the case.
Once you get
inside the airport (or if you are dropped off at the curb), you can easily get a wheelchair. But, if you are not dropped off, you will have to get from the parking area to where ever you can get a wheelchair. That may or may not be a problem for her.
We travel with a wheelchair all the time, so I don't really think it's that much hassle to bring one on the plane (plus we have no choice). You just tell the gate agent before boarding (well before boarding) that you want to gate check a wheelchair. They fill out a special gate check tag and put it on the wheelchair. Then, when you board, you leave the wheelchair at the door of the plane. When you get off, the wheelchair is waiting at the gate for you. It's not that difficult (people do it all the time with strollers). You do need to take a bit of time getting off or you will beat the wheelchair (but the same thing could happen if you arrange for the skycap to meet you with a wheelchair).
We did not travel on DME, but others in our party did. If she has a wheelchair, you will have to wait for a lift equipped bus (people have posted it was maybe an extra 10 minutes). Then she could ride in the wheelchair up the lift to get into the bus. If she doesn't have a wheelchair, it is (I think) 3 steps up to get into the bus.
If you plan to get a wheelchair at the parks, she'd need to be able to get from your room to the bus stop and then from the bus to where wheelchairs are rented. If you have a wheelchair with you, there is information about traveling on the buses with wheelchairs in the disABILITIES FAQs thread. We have no choice since DD can't get out of her wheelchair, but someone who can fold their wheelchair and transfer to a seat could do that and it would take less time.
There is also information about Guest Assistance Cards (what they are, how to get one, where to get one) in the disABILITIES FAQs thread near the top of this board. GACs generally are not given out to people with wheelchairs/
ECVs unless there are other needs besides a need for accessible lines. Just seeing the wheelchair/ECV lets the CM know that you need an accessible entrance and you don't need a GAC to use the alternate entrance if the regular line is not accessible.
In about 17 years of travel with a wheelchair, we have very occasionally had a problem - the most silly was at Splash Mountain where we got to a chained area with a wheelchair symbol. The CM would not let us thru because she said that she would be letting us "skip" too much of the line.
At that point, the regular line goes up a flight of steps and I asked her how the wheelchair was supposed to go up a flight of steps. She was not willing to budge, so we asked for her manager, who said the CM was being ridiculous because wheelchairs can't go up stainrs. The original CM continued to argue, but we were allowed thru. That is the exception rather than the rule and asking for a manager should always solve the problem (for what it's worth - we not only had a wheelchair, but also had a GAC, since DD has many problems in addition to using a wheelchair).