I know what you're going through, my 2nd GN was bought via a classic car retailer and had a similar issue but with rod knock that you only noticed on dry start up. Got nothing due to as is BS.
Take it from someone who's had others F up their stuff and get away with it because the drama isn't worth it, you eventually need to stop being a doormat. Even if you don't recoupe any $, don't let them think they can get away with this and not have any smoke come their way, add stress to their lives too.
Winning isn't just about your money, you can hurt THEIR future $ by being a loud voice to warn others from buying from them. This is how you'll crush them in the long run, the better you do at online posts, create a website even that documents the whole ordeal, BBB filing, actual court case that shows you won etc, hell even do physical marketing, you can dry up the well and then they'll have to sell or close shop.
Proof and a disciplined long game will take them out, keep track of everything and run things by your lawyer if you wanna start up the marketing of "Cons and Cars" campaign.
A bit of a different approach...
Label and arrange a display of the failed parts in the trunk. Complete with a detailed printout, and with handout copies of your story.
Put a rooftop sign on the car with temporary strap on beams, calling attention to what prospective customers may encounter. Park in ft of the stealership.
We had such a mess when I was with Chevrolet. The customer had so many issues with his pickup that he rode around town with a sign in the bed, complete with lemons drawn all over the vehicle.
The dealer bought it back.
These clowns won't care WHAT he does. They got their money, there's no looking back. If he does what you suggest, they will just pull up stakes , move to a different location, under a new LLC name, and carry on, just like before. I hate that, but it is what it is, and they know the game better than us.
They will get away with unscrupulous business practices, just like Paul Castle has and does, forever.
Hate to have to say it again, but, Let The Buyer Beware!