Seems you have done the most obvious checks,
I think its time to seriously think about a posible rod bearing, or main bearing has spun, it could have happend due to excesive heat build up, and when you turned the engine over after the hard run, one or more of the crank bearings had stuck to the crank, and spun with it! I used to work in an engine shop many years ago, and have seen most things!
Could also be a rod bolt loose too.
Just my ramblings, either way, hope you get it sorted.
A...