Well from the sounds of it the account is already gone. So I would complete what is on your contract and walk away. They will more then likely refuse to pay in the end. But just make a note of everything that you did and problems you have encountered, and how they were handled by the builder/GC. You will have no problems collecting from a court.
But for the next contract, be sure to add that fixture placement clause to your contract. [Linked Image]

James