Not sure if I have the best advice but can try. I've done two trades with a truck built to order. One was a Tundra trade-in for a Ram and then the most recent King Ranch for my Raptor.
The first one wasn't too bad. Dealer had to look at my Tundra and told me that I had to keep the mileage within a certain amount and it needed to be in the exact same shape when I finally traded it in. Only thing that happened to it was a crack windshield and I got it replaced beforehand. We bickered back and forth on the final numbers via email before placing the order but once done at time of delivery, it went by the numbers, no surprises and was on paper. I put down $500. Very happy.
Next one was a lot more sketchy, this was with Louisburg Ford mind you that quite a few folks here got theirs. And to be honest, I don't think I would go back after all said and done.
I had to leave a non-refundable $1000 deposit just like you. It was in writing that it was non-refundable regardless how enforceable that might or might not be. I was out of state but sent them pictures of my trade. I even offered to drive it to them so they could see it. They refused every time and said it was not necessary which I thought was very strange. They told me that the trade amount was "Possible trade in 2011 KR est trade range $29-$30k" and put that in writing on the Purchase Agreement. I told them I wanted more than that but was told over and over that they don't like trades and that was the best they could do. Mind you, it said "possible" so trading it in was optional, not a requirement of the deal.
As we got closer, I told them that I needed a FIRM trade-in price before I drove to them when the Raptor came in. I did not want to be in a situation at the signing table with a lower ball figure thrown at me and felt like I was stuck after waiting 5 months. Louisburg were jerks on the entire trade thing and it was pissing me off. Started saying that my 6.5' bed was going to make it hard to sell and they would have put tires on it, blah, blah, like they were doing me a huge favor taking it off my hands.
A member here tipped me off on a dealer he worked with that gave him a good trade and provided me a name. I called them and sent pictures, even asked me if it had any body work, which it did but Louisburg never asked that question. He gave me a price for $31k and would cover shipping to pick it up from Kansas, think he was in Michigan. Told me upfront that the pictures were good enough, his shipping was going cost him about $500 and he was going to sell it between $34-35k. It was a done deal and smooth. We were going to do a dealer pass-thru. Were I trade it in to Louisburg and then they turn around and pass it over to him. Why wasn't Louisburg like that? Jerks, that's why.
This is what I am suggesting to you. Get out to dealers and start a bidding war per say and definitely CarMax as suggested already. Do you owe anything on your trade?
My final outcome? Well I presented the pass-thru deal to Louisburg. At first they were offensive. Told me they were going to charge me $300 to do this. I told them it was worth no more than $50 in paperwork, they *******. I finally agreed to their stupid $300 as it was still $700 more than their best offer and it was a certain amount, no surprises. Louisburg came back and offered me now $30,700 for my trade and said they wanted to keep it with them but still would not guarantee it but I did have all this in email, if that even really matters. I asked them why not the $31k like the other and they played the well, we are going to charge you $300 so it's the same thing. I proceeded to move forward and continue to wait for my Raptor.
On delivery, they finally saw my trade and were kinda stepped back how clean it was and they didn't even have to prep it. While I was there, they stuck a Sale sign in it and move it to the front of the lot, it was that clean. During financing and trade, the guy ask me to sign the paperwork that there wasn't any body work. Told him I would sign it that there was and why I was being asked this question NOW. Getting pissed because I knew what was coming next. Told them they other guy asked that question and still did $31k. Louisburg stuck to their $30,700. But what a pain in the ass. Mind you, that body work was not nor would be on Carfax.
Personally I think you waited too late for 1 week before and should have gotten better numbers long ago, but that is the past. I would definitely get out there and start getting prices and even call out of state dealers. Most dealers are jerks and it doesn't surprise me one bit that they are trying to pull something like that on you. Contact the GM and see what can be done but don't be surprised if he/she is a bigger jerk.
Wish you the best of luck. I hate the whole thing.
EDIT: My brother owned his own car lot for 20 years and knows people. I have someone I can contact and he can look up what auction prices through Manheim have been for your corvette, regionally and average USA. PM me the entire details of your car and I will see what I can do.