From what I can tell, Fantasmic is at 8pm during your visit. There will be a huge crowd of people leaving the park when it's over. I don't remember how long the show lasts, but I'm thinking it's maybe about 30 minutes?? I don't honestly see how you can get from one to the other fast enough. The quickest way would be to walk from MGM through BW to the International Gateway. The walk in normal circumstances may take about 20-30 minutes depending on your pace. (There are boats, but they're much slower and you usually have to wait around for them.) You could maybe get out of MGM and get to BW in time to see the fireworks from Illuminations from the bridge. I think they go at about 9:05 and 9:12 or so(????). The are 2 sets that you can see from the BW. The only way would to get to Epcot would be to leave Fantasmic early and run all the way to Epcot.