If you call back and tell them you need first floor at OKW "For Medical Reasons", those requests are filled first and are pretty much guaranteed. If there is time to get a new confirmation sheet back, it should state something about first floor for medical needs. We have not ever had a probelm with getting first floor when they knew it was for medical needs.
As teri and Judith said, you don't need a GAC to take the
ecv or ecv into any lines or wheelchair seating areas with you. That's Disney policy. Most, but not all, rides have an exit pretty close to the entrance. You might want to rethink going into shows without it though. Some of the shows, like ITTBAB, have quite a long way to walk to get from the entrance (even the fastpass entrance) to the actual seating area. Also, most shows have an entrance on one side of the building and the exit is on another side. It's easy to get turned around and not be able to figure out quite where you are in relation to where you left your ecv or find that the exit is farther than you can walk to get back to the ecv. Before you go into a show without your ecv, you should ask the CM at the entrance where you will exit, how long the wait in line will be and how far the walk is from the entrance to the boarding area.
One DIS poster said she was asked for a letter from her child's doctor to get a GAC, the child had autism, so there was an invisible disability in a child who did not appear to have any problems. No one else has posted that they were asked for a letter and many people have posted that they tried to show a letter to the CM at Guest Relations, but were told it was not necessary. I asked a CM (told them that I was a Moderator on a discussion board for people with many kinds of disabilities) and I was told that a doctor's letter was not necessary. They said that usually when people have one and insist on showing it to them, it isn't useful because the doctor generally just states the diagnosis (in medical language with no translation) and there is no information about what types of assistance people need or things they need to avoid. If it's easy to get one, I guess it is a good idea to have one in case you are asked, but chances are that you will not even be asked about it.