Put the bike in the car. The seat mast shouldn't really matter. You mentioned S vs XS frame, so either way, its not going to be super tall post. On any car with folding rear seats, you should be able to remove the front wheel and fit no problem. If the rear seats don't fold, you can probably remove front and rear tires and toss it in the trunk (or hatch). If you can't fit a S frame in the trunk...well +1 for new car.

I do have a small cable lock for my bikes. I mainly use it to secure the bikes to a rear rack, so I can do things like stop for a quick lunch.