Everybody was shot on the Day of the Dead because that's the day when the events of all years take place. At least the first two, but I don't see why the last two would be any different.
You aren't being a snob at all. Your post included some info that I hadn't known before (like the meaning of flowers in Mexican tradition) and everything related to Grim Fandango is worth knowing. If you have any other details, go on and share them with us!