It's possible, but there is some risk. Fantasmic is about 25 minutes long. You'd plan to watch from the back of the theater so that you can be one of the first people out immediately at the finale. Then, the walk from DHS to Epcot will take about 25 minutes (maybe under 20 at a brisk stroll). You'll definitely want to walk, because the water taxis make several stops before arriving at Epcot. Driving subjects you to unknown risks, such as traffic.