There is no law regarding returning a car. No 3 day return policy. Unless the dealership put it in writing for you. And even then it might not hold up in court.
There are ways of getting what you want but I would not start out threatening anyone. Most GM keyless entry fobs can be programmed for a few different vehicles. Your best bet is to see if they have a extra one they would be willing to program for your car.
A salesman is going to tell you what you want to hear to buy a car. If it was owned by only one rental company and they are the second owner they are kind of telling you the truth that it is a one owner car.
I am with Jones, but my beachfront property is better then his it only has 1/2 a owner.