1. Actually, I just had an MSEP FP on Friday, and stayed there to watch Celebrate the Magic. I planned to watch Wishes, also, but there were technical difficulties with the audio that delayed it almost an hour, and I couldn't stay. (Don't worry -- I have seen wishes probably 30-40 times, and this is the first time something like that happened, so it's not likely.) It was a fine view of CtM and would have been a good view of Wishes, too. Of course, they take the ropes down that define the FP area after the parade passes, and it does get a bit crowded.
2. Maybe a little, but it's not significantly different from closer to the castle, in my opinion. Pretty loud either place.
3. It passes by the FP area in about 20 minutes.
4. What do you mean it "closes" at 8:00 pm? What day are you going? Is that the time that Wishes starts?
With your child's potential issues, to be honest I would not use the FP. I think it would be better to have more flexibility to move around and choose where YOU think is best.