Flow some excellent points there. I would say is what we all don't know here is if the horse has already bolted in terms of DS's past assertions of authority (if any), as such, if OP is looking at damage limitation, or still has power to lever damage avoidance altogether.
You have assumed it's damage limitation that is OP's option left, in which case your suggestions are great.
I have assumed there is still a horse in the stable for the door to be bolted :) in which case I wouldn't react in a damage limitation kind of manner.
However, it is extremely concerning, if the 15yo can trump everything, every discussion, everything they don't feel like doing/don't want to do with "you can't physically make me". None of us here know the OP's DS or how he is or what battles have already been fought/lost/won but if this is the first real, big challenge then the OP has a big decision to make in terms of direction of authority. Only she can know if her DS is mature enough to be trusted at 15, or if she should hold out for longer - until he's older, on a matter more sensibly raised, with time for proper negotiations and arrangements to be made.
I do understand that in some personalities it's unavoidable that this will happen - I am not smug about behaviour - I am well aware that what other children are exhibiting in bad behaviour today, mine can well be doing tomorrow. I have two sons, one compliant natured; one, umm strong willed. If I let either of them get a sense of their own power that all they have to do is (ultimately) say "you can't make me (physically) " then it's dangerous territory. It is terrible to think that the psychological relationship had been lost to such a point that there is no armoury left in words/threats/loss of priviledges/assertion of authority from the parents that will do as you are saying would be ideal, ie, them getting in the car of their own will.