[unfoundnemo]

I don't think you're overacting at all if you had a warranty that carried you to late 2019 and had noted the issue before. Do you have the service records where the SA noted your complaint of consuming oil from before the warranty period ended? If so, I would think you have a fairly strong case to argue they should be covering it under warranty.

The 12k carmax offer is quite a bit light, but you're certainly not talking about a 13k difference from what the trade in value should be. Perhaps get the dealer's trade-in value for the car as-is now, then bring it up to the GM so he at least has two options (give you more for the trade as-is, or help to make sure they get the claim covered under warranty so they could give you the full value).

If you do decide to walk, you could likely get another dealer to intercept your build so you could still get the same car in the same time-frame.

Good luck.