Well, the tires were purchased (and mounted) at Costco, and rotated at least once previously by Costco (since the path of least resistance for warranty maintenance is for them to have the records of doing the (free rotations) themselves; they'd have a hard time arguing in the event of a warranty claim that they'd rotated the tires incorrectly).

So history argues in favor of it being their fault. In fairness, they were apologetic and never tried to absolve themselves of responsibility.

Regarding "Costco should have had zero problems replacing a broken stud on the spot", I wish it were so (would have saved a bit of hassle), but which expert person at Costco is going to be capable of doing this? They seemed to have shop nearby that they farmed such work out to, but this problem occurred on a Sunday...