Yes, you can watch fireworks from the beach. Was just there and the beach audio quality and volume is among the best it has been in years.
Unless you want one of the Adirondack chairs that are on the beach, I don’t think you need to really be all that early before the show. 5 mins or so. If you want a specific spot, then go earlier. But it’s easy enough to wing it right before the show if needed.
EWP is currently around 8:50 - so people show up for that and tend to stick around for that (although fireworks and EWP times can move around during others times of the year).