I don't want to beat a dead horse here, but I'm going to beat it just a little bit, because I think this is really important.
Do you know this for a fact, or are you just assuming?
Because again, I am not an attorney, and have no expertise in this in the US, and certainly not in Canada. But it seems to me there is a major distinction in a company having insurance and liability coverage that covers them working on a customer's car that is insured by the customer, where their coverage is supplemental to that coverage, and what was taking place here. There is no doubt in my mind that a) Tesla has coverage to cover cars Tesla owns before they deliver them to customers, and b) Tesla has coverage to cover a customer's car should something happen to it while under Tesla's care. But my concern is that the b) coverage may well be contingent upon the car being registered and insured by someone else, and if it isn't, no coverage may be in effect.
This was a bizarre situation.
I don't think Tesla's attorneys and insurance people try to write insurance policies to cover bizarre situations like this, nor should they. I think they probably count on their store personnel generally not getting them into situations like this, and I expect by and large they don't, and everything is usually just fine.
My point is that unless you know for a fact that Tesla carries some special kind of coverage that would cover them in this situation, I don't think you can just assume that the same kind of coverage that would cover your regular auto mechanic applies. It's not the same situation at all.