Flying in ashCLOUDS is definetely unsafe.
But the discussion is whether flying in contaminated air by ash is safe or unsafe.
The testflight Dornier did fly in clouds.
The KLM, LH, AF, BA testflights stayed VMC and had apparently no damage.
So why not open airspace during daylight and VMC only. ?
Climb VMC to FL200+++ and then overfly the contaminated air.