Hi and welcome.
I saw your post yesterday, but I had to get off the computer then and didn't get back until now.
Besides the hints you already got, I suggest you plan to go at a time when it isn't traditionally that busy, if you can. One way to tell how busy it's expected to be is to look at the prices for WDW hotels.Avoid "Peak Seaon" (which is Feb 14 thru April 20 for 2002) and "Holiday Season" (which is Dec 20 thru 31st for 2002). The slowest season is usually January, September and the first half of December.
If you go then, you will find it's a lot less busy (less stimulation and less crowds to worry about). We went a few years ago in Sept and found there were few waits in line and places where we barely saw anyone else.
You may find that your DD will not allow you to use a GAC (another sign of being different). I guess I'd still get one, just to have in case you need it. If you are at the parks for a few days without one and then decide it will be helpful, you can go to Guest Services in any of the parks to request one. Going at a non-busy time, you are much less likely to run into situations that make one necessary.
The other hint I have is to use fastpass as much as possible. You get a fastpass and then return to the ride/attraction later and have a 5 to 15 minute wait in line.
Here's a link to fastpass info in case you don't know about it.
For the mobility, you will have to decide if teri's suggestion about getting a wheelchair, just to have, would be useful or not. I know in our case, my DD has cerebral palsy and a "from birth" brain injury. She gets "stuck on things" (I hope you know what I mean) and she would get stuck on a strange wheelchair. She has her own wheelchair, but there are times when she can't use it (like at the Pirates ride and if we have a tight connecting flight at the airport). She will agree to use the "different" wheelchair, but when the time comes, she won't do it. No way, no how. Logic doesn't help. It's just that she won't and it would definately freak her out to even have it by her.
Another thing to think about for dealing with the memory parts, is the think back to tools she might have been taught in her brain injury rehab. Some people use little notebooks to jot down things they need to remember. The act of writing it down helps to imprint it into the brain and then you have it with you too as a reminder. Think of situations that might need some decision making and help her make up a little notebook (maybe one for each park). I don't know if you have ever been involved with a local
chapter of the Brain Injury Association. This is the the one for Minnesota, but I know they have a lot of helpful info. I couldn't find a link on their site to the Brain Injury Association of America, but I did finally find their site.
Here's a link to the page that has links to individual state programs. You might be able to get some hints from support groups in your state.
Good luck and have fun.