We've gotten into the habit of finding a bench somewhere, grabbing a coffee or hot chocolate. There's a coffee cart between Canada and UK that stays open for awhile and there are some benches nearby. Relax, enjoy the music (which continues for at least 30 min after Illuminations is over) and watch the people leave. The crowds will clear out from around there pretty quickly, and by the time you finish your coffee, the population density will be approaching "empty" in that area. Exiting via either gate will be simple.
Steve