Unless the user gets in an accident, is there a way to actually prove the frame is structurally defective?
Tesla's service center might be willing to say "within spec!" but I suspect they'll have a much harder time finding an engineer willing to put their license on the line in court.
A lawsuit. That is what the legal system is for. But for some reason people seem to look down on this which I find baffling.
Including Lemon Law claims.
In my state, you must start with arbitration but you have zero obligation to accept that decision and can sue regardless of outcome.
If you think that I am wrong, please explain why.
(You are right that you "can sue regardless of outcome," but only in the sense that anyone can sue anyone for anything. If you enter into an agreement to arbitrate disputes, the outcome of that arbitration is generally binding.)
It probably wouldn't get to court, though. The government deals with lemons all the time. Every state has ways to get lemon buyers relief quickly and at minimal expense.
I think the photos do that just fine