It appears that the dealership is the one on the losing end of this deal. Refuse to speak with anyone from the dealership from this point on as you may compromise your position. You have a binding contract signed by both yourself and an officer of the dealership. If they can't do the paperwork right and they have gotten paid for the car it is legally yours and they have no right to withhold the title of your car as this equates to extortion.
I would contact a lawyer as well as the consumer protection branch of your department of motor vehicles to report the dealership for harassment. I would also attempt to involve the legal representatives of the credit union because they are also a party in this situation since they hold a lean on the title. If it has not been filed then they are at risk of losing the money that you borrowed to purchase the car.
Finally, you have a signed contract the has been properly executed. Once this gets to court, if it goes that far, the dealership will lose because the error was theirs not yours. Good Luck and keep us posted.