The night I saw fantasmic, the RBT closed the outside seating area sometime before the 2nd fantasmic show. I know this, because we went there to eat, after seeing the first fantasmic show. We were going to sit outside, and found it was roped off, and we were not allowed to sit there.
We listened to the fireworks sound effects (music and bangs) while we ate.
The next night we saw the fireworks from a spot where we could view the front of the castle.
I think the DL cast likes people to just stay seated, for fantasmic, to view the fireworks. But so much is missed (at least from the ground seats right by the water, maybe the balcony has a view of the front of the castle).
It is almost impossible to move very far, after the first fantasmic show, while the fireworks are going on. So we decided to eat and then escape the park later, and see the fireworks the next night.