While I haven't sailed the Wish, all my DCL cruises were early dining waitlisted (13 cruises). Only 2 of them didn't come through.
Regarding the fireworks and late seating, they schedule it so you don't miss the fireworks due to late seating. Unless you're a real lollygagger and don't finish eating on time. If it makes you more comfortable, and you don't wind up on early seating, you can ask your server to make sure your dinner that night comes out fast so as not to miss the fireworks.