I called Audi FS and was told “if tires are needed they will charge you the dealer’s list price for the tires and labor; they don’t shop around for deals, so assume if a full set were needed it could be $1200-$1300.”
So even if they found nothing wrong with the car aside from the worn tires (which would be the case as the rest of the car is immaculate) I’d still be on the hook for potentially $700-$800 after taking the $500 “forgiveness”.
You do make fair point about the registration. Hadn’t thought about that. Renewal in NJ is about $50 though so I will have to just eat it and know I paid for 9 extra months of registration.