AnFi what are the alternatives?
AH currently has a multi billion dollar program near death. In that context, blowing up a hundred gear boxes would be a cheap solution, if it helped restore the type to market acceptance.
The logic for the planet gear reliability calculation is yours, but note that we had 2 catastrophic failures in about 2 million hours of operations iirc, so we are a long ways from 10 to the ninth.
I'm no regulator, but I'd think that the acceptable rate of catastrophic gear box failure is zero. I'd hope regulators would not be bamboozled by sweet talk about 9 sigma reliability. We don't tolerate aircraft whose wings fall off either.