We will continue to debate this til the cows come home. There will be no resolution until there is something in writing from both the manufacturer and the regulator that clearly states a policy.
Looks like the debate is over, then. Boeing tells you not to fly into volcanic ash clouds. Eurocontrol and several member nations published a clear policy that prohibited IFR flight into areas they deemed unsafe due to the volcanic ash cloud. They assessed the risk and decided it was too high for continued IFR flight. That is the basis of sound risk management when it comes to aviation safety.
There will always be second-guessers. The fact that there were no ash incidents gives clear evidence that the Eurocontrol policy was successful. There is no way to tell how many incidents or mishaps there would have been without the policy.
Airlines now want the governments to pay them for the money they think they lost. What would have been the cost of even ONE crash due to ash injestion? I think the airlines may have saved money...