1. It depends on your jurisdiction. Often they are not clearly defined. In part because there are so many possibilities and it's difficult to write rules to handle all of them. This is where judges come in. If you go to court, it will depend on the judge and their take on the situation.

That aside, my original point stands. You sign a marriage contract and the state legislature can change the terms of the contract without your consent.

2. Could you elaborate? I couldn't understand the point you made. As far as I'm aware you don't sign a contract when you drive a car.