from the Hall of Presidents at MK to be the place for the afternoon parade as my favorite location. You do need at least an hour ahead to be in the front row. I find the other parks for the parade to be pretty easy, as long as, you allow the hour's time for the parades at AK and HS. At AK I like the handicap area by Africa the best for the afternoon parade. I don't have a favorite place at HS, but we seem to end up near the Sorcerer's Hat a lot.
For fireworks, MK can be very tough if you want to be in the front area of the castle. There's only one handicap area you can really use; I would allow at least 90 minutes for a front row place. If you don't care about the MK night parade you can go up a ramp at Crystal Palace and it's a pretty good view of the fireworks if you get up front at the rails of the porch. Some go to the MK dessert party (you pay extra) and that's a pretty good view, but again you need to be up front.
At Epcot for Illuminations I find I like the Italy handicap area as my favorite. I think 45 minutes ahead of time would be fine there. There are trees there that can block your view somewhat.
For Fantasmic at HS, we find getting the meal package with a special seating pass included as the best option. Otherwise, you may need 90 minutes to 120 minutes there. There's a very limited amount of handicap places for Fantasmic.
It's tough to do the parades and fireworks with a wheelchair or
ECV. We have skipped them at times and enjoyed the rides and attractions instead.
You can view the parades or fireworks without going to a handicap area, too. Actually, if you plan to get back to Disney in the future without the ECV I would skip most of it on this trip.
I have to say it can get rather nasty at times at MK in the front, handicap area for the fireworks. I had my mom, who was 89 at the time and in a wheelchair. She had Alzheimer's in the worse way and a high level of separation anxiety without me; I was her security blanket when we went out. Anyway, another adult couple wanted me to move to the back when it was time for the parade and fireworks to start. We had talked to this couple for at least an half hour waiting for the show to start and it all seemed very cordial; until, it was time to see the show. I had no room to sit anywhere, either. We placed someone's child in front of my mom's wheelchair, already. This other couple really made a big thing out of it. I did my best to explain and then I decided to just ignore them. It did make the experience stressful. It wasn't so magical that night. I believe the parades and fireworks was one of my mom's favorites going to Disney; so, we made sure to do them, but it can be tough.
Another poster, here has a so-called secret spot at MK and sometimes the poster will share that secret in a private message. Maybe, you will be one of the lucky ones to get the secret place.