This info on steels, bearings, etc. is excellent.
But isn't it time for a reality check? It seems clear that the combined talent of the design authority, crash investigators, regulators and operator maintenance staff is unable to fully understand or control what is happening to these aircraft. Whatever it is that we know about materials, operating conditions, wear rates, etc, that knowledge is inadequate in this case.
I know that the aviation sector is generally run with great maturity and care by all those involved, and takes pride in knowing all aspects of aircraft behaviour and how to operate safely.
However for this type they imposed a regime of intensive examination after every flight, after a few other accidents occurred. To me that sounds like "we don't really, truly know what's happening, so we'll sample the gearbox condition at every opportunity and maybe find out". In retrospect that's probably going to look like a bad idea.
The most important part of the investigation will be its focus on how it came to this.