[Pyrotechnic]

Thing is, if the head gasket wasn't blown to the point of being burned, but just leaking enough to cause an issue they very well may not have seen it and just thought it was some sort of customer error causing the overheating.

Even if they did find a blown head gasket, it's not like it's a huge cost for them to fix it. Not like you sent them a grenaded bottom end or anything. Can't see why that would be such a big deal just for them to fix and leave a customer happy.

Doesn't make sense why they are trying to stiff you on this one, a customer stiffed out of this much money is going to talk, and at a very loud volume to a lot of people. That's very bad for business.

I would ask to speak to the company owner or someone higher up if you haven't already. I've been through a similar deal by a local builder, I hope by some miracle you get this all worked out.