Nationwide Mutual Insurance

I have a 2014 SRT Viper that was vandalized or damaged by an unknown party. The vehicle was driven from Texas to California on August 25, 2018, at some point during that trip while the car was left unattended, I believe someone threw or poured sand in and near the oil filler spout. The vehicle completed the drive successfully to California and parked in a secured garage of my friend's house. The car stayed there securely until I returned on October 23when I attempted to start the car it would not start, so I contacted AAA to try to jump the battery, which was unsuccessful. I had the vehicle towed to A& C Performance in Temecula, CA where it was diagnosed that the engine was seized. After a complete tear down on the engine, it was diagnosed that sand had damaged the engine's internal components. I immediately contacted Nationwide to file a claim and gave all the information I described above to Mackenzie Hewitt. I was then contacted by Stephanie McClain who was the assigned appraiser. I gave her all the information that was requested and asked if any police report or possible video footage was needed, I was told that she would let me know if that was needed.
The next time I heard from Stephanie she explained that her manager Aaron was looking into the claim and he would be contacting myself and A&C Performance. After a few weeks, Stephanie called and said the claim was approved and she was waiting on the estimates to get them submitted. I was then informed by A&C performance that Aaron had called the owner Todd and was hoping to send over a check the following week.
The next point of contact was when I called Aaron to follow up on the payment and he explained to me that his supervisor - Pam had requested a 3rd party inspector to verify the cause of engine failure. I waited another 3 weeks to hear back from Aaron and have him explain that the claim was being denied because the inspection did not find evidence of sand. He then emailed me a copy of the report the inspectors had completed which contained fictitious information and were missing evidence and photos that were taken by A& C performance on the first inspection of the engine.
I believe that Aaron has taken an extreme amount of time and delayed this claim in order to provide a falsified inspection report and allow time-sensitive data to be lost, (any possible video footage that could have been used as evidence) He has shown an arrogant attitude to my loss and caused me both financial loss and anxiety that was unnecessary.
I am demanding that the claim is reopened to allow for proper investigation to be completed and a 3rd party inspection by an SRT Viper certified technician. I have researched the integrity of the inspectors that Aaron hired, and they have a high amount of reviews that state they are loyal to the company paying the bill and do not deliver accurate inspections.
I feel that this claim was denied without proper investigation and Aaron is responsible for inaccurate information and unnecessary time delays.