I watched the fireworks from the Grand Floridian and I think I know the spot you are talking about. They pipe in the music on the veranda of Narcoossee's restaurant. The little hill would be the grassy area just to the left of the restaurant. It has a clear view of MK and the view is great. They also have benches to sit on at the boat dock but I couldn't hear the music very well from there. I prefer the grassy area next to Narcoossee.
I've also watched the fireworks from the marina area in front of Gasparilla's. This is a nice area, too, because there are a lot of tables and chairs to sit at and they pipe the music in there, too. The fireworks view is pretty good but you can't see the widest fireworks. But it is a nice place to sit.
Anyone can go there.