You Do Have Comprehensive Insurance, Right ? Should You Find " . . . something along the lines of the fuel being tampered with, . . ." You May Be Covered By Comprehensive Insurance.
The dealer mechanic would have to write a statement on the estimate repair order that the fuel was tampered with (vandalized) and that it caused head gasket failure, ecetera.
It would be up to your car insurance company to either pay the bill (less the deductible) or send a claims adjuster to discuss the situation with the mechanic. I would say that if the adjuster shoots them down (or they chicken out) that they would have to go the warranty route. Some companies just have agents write a check for any claims under a certain amount, like 1000 bucks.
Seems to me like it's one or the other. Either it was a parts / manufacturing defect or it was vandalized, but not both.
Do you have comp ? Do you have an "E-gent" or a brick and mortar agent ?