We have 2 sons with cerebral palsy who are in wheelchairs, so we've done the Fantasmic and Fireworks things alot. We did WoC once last visit, and it ended up working pretty well for us. But, there had been a lot of threads about WoC and wheelchairs, and some of the problems other folks had, so we were pretty well prepared.
1. Fireworks at the hub: not to be discouraging, but this quite frankly, is the worst part of our vacation-time at DL or at WDW. There is NOT a dedicated handicapped section for the fireworks. And, since everyone stands in the hub, it is extremely difficult to keep a good line of sight for someone who is sitting in a chair or ECV. Yes, the fireworks are up in the sky, so it shouldn't matter about sitting (that's what Disney says when they explain no handicapped seating). But, so much of the show is projected on the castle, and even up with the fireworks up in the sky, they're actually behind the castle, so if someone stands in front of you and you're in a chair, you won't be able to see much. Our best bet has been to find a place in the hub, and park the chair up against a railing. There may be some trees that obscure some of the view, but at least you won't have someone who at the last minute will stand right in front of you and give you the great view of their back all through the show! To get those railing spots we often line up 2 hrs ahead of time. Because of all the hassles, we only do this about once every 2 yr.
2. F! The handicapped section there works well. It can fill up, but they are very good about letting chairs up in the front, and the other well-bodied folks in the party can stand behind. It is off to the side, so you don't get the best front view of the show, but it's still very good. You don't have to be there as early, but if it is a very busy night, you still want to be there at least 1 hr before hand. You can also camp out and park chairs at the back of the F! viewing area, where everyone ahead of you has to sit. But, you have to be there 2-3 hr before the show. You were asking about the 2nd Fantasmic after the fireworks. With the kids, we haven't done this, but you should have an easier time finding spots.
3. WoC. I would suggest you really look at that superthread and another thread in the disABILITIES board about WoC. We got there early, in order to make sure we were in the "front" of the little tier, and sure enough, they packed folks in there. There were people in wheelchairs with people standing right in front of them, with no ability to see whatsoever. And, then all the folks even in the lower tiers who had climbed on the railings or with kids on their shoulders, it was still a tough place to watch a show in a wheelchair. We thought it was amazing, and we will go back, but we will make sure we do it like we did this time and get there very early.
Hope this all helps! Have fun!
W