All incidents in the past occurred in thick, high-density ash clouds. NOT in nice VMC weather.
This is Northern Europe. Where can you ever guarantee nice VMC weather here ?
Well, military A/C should be flying in the red zone by day VMC, actively looking, but avoiding visible ash concentrations. Make a post-flight inspection of the A/C + engines. No damage? Take-off again and go look for some visible (pollution-like) ash and examine the A/C after the flight.
Until now there's NO evidence that ash > 4000mgrs/m2 but still invisible to the eye has ANY safety impact on an A/C.
Why military aircraft ? They don't have any sensors to determine the level of contamination, therefore although they might prove damage, it doesn't assist anyone in determining what level of contamination or type of particle caused it. It is this figure which is surely important to the industry, as it will allow the tolerable level to be enshrined in procedures. The only way you are going to get that information is by using Met Research aircraft with suitable equipment on board, and a suitable ground based team to assess the data and make the judgements for the Regulators to act on. Maybe the manufacturers and the airlines could pay for this to happen urgently as it will ultimately be in all their interests. There certainly doesn't seem to have been much done in the last year to advance the publication of a 'safe' level.