Don't forget that you may not be able to hear detonation taking place especially with a loud exhaust, and heavy detonation can do a lot of damage in a very short time.

I'd think that char on the underside of the piston is going to be a factor of general chamber temps -- detonation doesn't normally take place in the middle of the chamber, but high temps from CR/lean mixture running will be experienced by the piston there.

https://help.summitracing.com/knowledgebase/article/SR-05007/en-us

mentions: How is it indicated?
Knocking or pinging sound
Drop in Exhaust Gas Temperature (EGT)
Broken piston rings and/or spark plugs
Damage to the piston and/or valves

You've sure got broken piston and rings covered!

There are some photos of detonation damage here:

https://www.lycoming.com/tips/detonation

The Aprilia/Rotax valves are nominal 36/31 with 6mm stems.  Power is roughly 125hp @ 9500 RPM.