What to do if a dealership call the deal off after all documents have been signed and trade-ins delivered to them?

You have the right to enforce the contract and have the vehicle you purchased. Once the contract is executed by both parties, one party may only lawfully terminate or void it in the event that 1) the other party breached it some material way; or (2) the other party knowingly made a material misrepresentation. ("Material" means "important" or "critical" in this context.) If the dealership had agreed to accept the vehicles as they were, and you did not knowingly lie or misrepresent about the vehicles, the dealership will be obligated to go through the  contract, since you honored your obligations. That perhaps the dealership made a bad deal and should have inspected the cars first is not your concern.

So as long as  you did not knowingly lie, and you did what the contract and paid (including trade ins) what the contract calls for, then based on what you write, you should be able to enforce the deal.