IMHO $9K off is a good starting point. 9% minus another 4K off MSRP should work. The dealer will break even but eat the floor plan (interest) costs.

You may do better selling the Vette to a specialty dealer. I had great success selling to CarMax, do you have one in your area?

I would not be concerned about the check engine light but would want to know the code before it's cleared. Could be emissions related from brief engine starts.

Remember it's a new engine that will feel stronger over time.