A - you will have some time to explore, but probably not tons of time. We had a 5:30-ish reservation (at Tutto) for the 8:15 show a few years ago and by the time we were done with dining, we didn't really have any extra time. We walked straight down to the theatre and there was already a pretty long line for the package holders, this was around 7:30 I think, so we just got in line. Your reservation is a bit earlier, and if it's not as crowded that night, you would have some more time (or if your dinner doesn't take as long). But you wouldn't have hours to wander WS.
B - As for the fireworks, I have heard it's very difficult to make it over to the viewing area for the fireworks if you see the last show. I have no personal experience with that one, just repeating things I've read here. Others may disagree.