Check Engine Light On problems of the 2009 Mazda CX9

I purchased a used 2009 Mazda cx-7 in June 2012. In may 2014 my check engine light came on. I took the car to the nearest auto service station which was not a Mazda dealership and the engine stopped when they tried to turn it on. I had the car towed to a Mazda dealership and had a quote of $8,000 because I was told I needed a new engine. I purchased a 5 year/100,000miles warranty with american guardian in 2012 when I purchased the car. I filed a claim with american guardian and was told that the engine fail from lack of oil. I presented all my oil change receipts which I had changed every 3 or 4 months over the last 2 years. The auto service company also sent out their insurance inspector to inspect the car to see what caused the engine to fail and if they would be liable. Their report stated that the timing chain jumped which cause the "cam shaft retard" code. The mechanic did show this code to me on their computer before the car was towed to Mazda. I reached out to Mazda USA and they concluded that they agreed with american guardian and cannot help me. Now I'm left with an $8,000 bill that no one wants to cover.