No. Being late on a debt--or even not paying it all--is not a crime, so there is no theft or felony. (One exception: if you never intended to pay the debt at the time you borrowed the money, that would be fraud. However, as long as that was not the case, it's not a crime to miss payments.)

Also, the dealer cannot cause your plates to become invalid.

What they can do is sue you for anything you owe them. And depending on the terms or conditions of the car loan, they may be able to reposses the car.