Update: just got my car back from the dealer with 2 new seatbelts. They discovered that the factory fits the belts to the chassis before the body is installed… so it took them a long time to remove and replace. Glad I didn’t try to do it myself.
They also discovered one of the rubber mounts on the new stage 2 exhaust had already sheared through, after about 200km…
Anyway, back on the road for now!