Guilty! lol We love the Halloween parties and have gone to many of them over the years. I don't mind the questions at all, I'm happy to share our experience!
I would try for Main Street. They do projections on the castle, Matterhorn, as well as the buildings down Main Street during the fireworks. You'll want to get curbside seating here, then as soon as the last parade float goes by - quickly move out into the center of the street so that you can see the fireworks. The other option is staking out a spot right in front of the castle and then grabbing a spot for the second parade, but this eats up a lot of time. If you want to do the treat trails or rides, that option probably isn't the best one.
The area to avoid would be at the very end of Main Street right in front of the hub, as there are trees on either side blocking the view of the fireworks and projections.
I really can't guess at fireworks cancellations. I know they do get cancelled for what seems even the slightest of winds. We haven't been to a party that has had fireworks cancelled (knock on wood!), so that's good. Not to say it never happens, but there seems to be a smaller chance during September and October. I've been told that if they do get cancelled, they play them for the next scheduled night (so if you go to a Friday party, they'll have them as Saturday's fireworks instead of the normal ones).
Hope this helps!