IMHO, unless you're a pro who sees the show a lot, you don't want to watch from BOG or the behind the castle area. As
@CAS239 noted, you won't see any of the projection element of the show. Also, your head will be on a swivel between the larger shots behind the park and the castle fireworks. It's a cool/loud place to be for the fireworks, but it's not really the best show overall.
HEA will end at 9:18... if you're in the hub it's going to take you a bit of time to make your way behind the castle through the masses. But I think you'll be OK overall. Just keep swimming! It's going to be crowded.
FWIW, in my experience the path through the caste and the two immediate side paths around the castle that they wet down pretty heavily are closed right after the fireworks. Not sure when that area clears to open.... but I remember sometime earlier this year I was trying to go on the path to the left and the CM said I'd be better off walking all the way around by CHH/PP/IASW, etc.