PM Deysha at customer service on themustangsource.com. You have probably already done this.
When you say you had it in email and writing, was the writing a purchase order signed by the salesman? If so, your case is stronger than just a written offer. Of course the fact that no money changed hands doesnt help your case, regardless if the dealer was going to cash the check or not. It's becoming obvious that this SOB had no intention of selling you the car. I hope you can somehow swing it but honestly I'd just look for another offer.
Updates???