Back to OP's original problem with smashed spark plugs:
@drill_and_buck already asked but got no traction, so I'll ask it again:
How much inspection is appropriate here? Any possibility of cracks in the pistons? Or combustion chamber castings? I'd spend more than a few minutes with a borescope at a minimum, in ALL of the cylinders. If leakdown numbers are still good (and that's a good test to do too), I wouldn't worry too much about the valves, but inspecting the seats for dents is worth doing too.