KLM test flight
The test flight has only proven there's no immidate threat to aircraft flying through such dilluted clouds of volcanic ash!!!
Anyone with enough common sense could have told you before the flight.
Even after boroscoping these engines afer the testflight would not show anything but that doesn't mean it's all ok.
After longer periods of operating airplanes and their engines with these conditions it could very well have effect!!!!!
But the good news is that all aircraftengines are monitored via a health monitoring system.
So if after some time residue will start building up on some engineparts it will be picked up as the engine will start over time to run hotter and less fuel efficient!
So I think it should be a commercial choice to fly or not to fly as engines will last maybe only 3000 hours on wing rather then 5000 hours but there is no immidiate danger!!!