Yes. Everyone with a ADR that night can come view the fireworks, regardless of their dining time. Those that ate earlier can return, and those who are eating later can come early, and they view from the observation deck. You'll check in at the host stand on the 2nd floor and ~20 minutes before the F/W, they'll start sending the elevators up full of people.
Depending on when the F/W are scheduled, don't be surprised if you table becomes available quickly after the fireworks, most tables turn over by that point.
I've been there several times. There are a decent number of people but it's not super crowded. I never felt like I had a problem seeing the fireworks show.
We have been there multiple times for Fireworks and neither deck has been very crowded.
Looking at MK, we prefer the deck to the left, but both give you a good view and both skew the fireworks off center of the castle and both have music.
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