Flying into anything other than air has always been a user choice. The manufacturers simply provide you with guidance on recognizing and correcting any malfunctions that might occur.
The regulators provide you with statements of certified tolerance (birds, ice, hail etc.)
If these statements do not cover you then you are on your own.
Operating safely is a domain left to the operators who of course must not be found in violation of a regulation.
Guidance on assessing the level of safety for a specific flight may be developed in concert with all parties and as such would be approved under operational procedures.
The referenced documents from Boeing and Airbus above (IFALPA) are great starting points as is all the stuff about tracking red yellow and green (sic) areas of ash. But the final output from the consideration of this data rests with the operational experts and not with message boards.
IMO there is no way that anybody outside the loop of developing and reviewing thse operational procedures can make judgements on what is safe and what is not safe.