We met a family on our trip in August, whose son had DS. They had a pair of headphones for him, and said they had been a big help, as he was also pretty sensitive to loud noises.
While the mom and I were chatting about rides our DD had thought were too scary, she mentioned that they had wonderful character interactions at the meet and greets...especially with Mickey.
It really did seem to me, based on that conversation and on my observations, that the CMs (and, especially the characters) are very used to dealing with special-needs kids and will go out of their way to make their visit magical.
As for the meet-and-greets, if your DD is into princesses, they will be over by "it's a small world" for most of the day. Mickey and Minnie will most likely be in ToonTown. Your DD might like the Disney Jr. show over in DCA, too, if she watches any of the shows on Disney Jr.
And, I think the GAC for "stroller as wheelchair" is a great idea. It will save your back and arms and her legs, which will make everyone happier! In DL, it will mean using the handicapped entrance, which is usually the exit. In DCA, you'll just wheel her into the line.
Have a wonderful trip!