Email me your VIN# at
[email protected]. I may be able to track down an insurance claim if one was ever reported. This may get you pointed in the right direction which I can explain to you directly later.
As far as your warranty, they need to be able to find the problem first. Unless the coolant has air pockets, then I doubt a flush and fill would solve the problem. You can probably find a much better deal then $400-500 to flush and fill your cooling system as well. A question I have is if there is an actual cooling problem. OBDII codes can be triggered by faulty, or damaged wiring as well. Could have been damaged in the accident.
The technician should be able to determine if the sensor that's triggering the code is faulty or not. He should also be checking to make sure the ECU is receiving the correct signals(indication that the wiring may be a problem). A technician should not be recommending a trial and error method without some factual base at the minimum. Demand they locate the actual problem. They may have to charge you for the diagnosis, but you can usually apply that towards the repair. If they are intent on the flush and fill, ask them if the engine is actually running too hot and how they know this.
Honestly, I think you're being taken for a ride. A flush and refill is a relatively simple procedure and at $400 to $500 for it, they are making a lot of money for little actual work. Parts cost is pretty low too. I had a dealer try to get me to put all factory parts back on a car once just to rule out an electrical short. They gave up asking when I started to grill into them about how they think it even had anything to do with it. They called back and said they found the problem an hour later. And once they do find the problem, they can confirm if its actually an accident related issue or not.