Any general rules about how events start/when seats are given up? Is there a grace period before unclaimed seats are resold, etc.? Is it right on the dot of the starting time? 5 mins. prior/after? Etc? Or is every event-runner making their own rules?

The event host policy says:

Wait up to 15 minutes for late players, if you can. But if your event schedule is extremely tight and must start on time to finish on time, you can count ticketed players as no-shows at your event start time and accept stand-by players at that point.