Approximately what time will the Jonsi/Riceboy show actually start? We may be a bit later than the scheduled start at 8. Will we be able to be seated if we show a bit late?

I believe the bands/performers usually hit the stage very close to the time that's listed.

The seats are numbered, nobody will take your seat.

Yes you will. Your tickets have your seat number

It should start on time. An usher will guide you to your seating when you arrive. See all information about the event at https://copernicuscenter.org/jonsi-alex-somers/