I haven't torn it down and inspected it.....and don't plan to. But judging by the way the piston melted down, the white-hot deposits on ALL of the spark plugs, and the fact that the fuel in the vapor separator tested over 18% alcohol content, my guess (for now) is typical detonation from low quality fuel..
This happens when a glowing hot spot inside the combustion chamber begins to ignite the fuel mixture as the piston is still moving up the cylinder on the compression stroke. Also known as "pre-ignition" or "dieseling". Sometimes, as in this case, it blows the edge of the piston off so that the excess pressure escapes out the exhaust port. In other (worse case) scenarios it blows a hole completely out the side of the block.
This motor actually has detonation sensors that are designed to retard ignition timing if pre-ignition is detected.....but when you have a glowing hot spot inside the combustion chamber it doesn't need a spark from the spark plugs to ignite the fuel mixture....so detonation sensors can't save it.
Bottom line......Poor quality fuel, and a driver that wants to haulass. π