My 2cents. I'm not sure why there is a DE debate at all. DE is ubiquitous in the soil where I live so worrying about its safety seems to be a waste of time. I literally live in the stuff. There is some lady online sponsored by some mite insecticide company (of course) that claims DE is both carcinogenic and ineffective against mites. This is rubbish considering DE is not even on CAs prop 65 list as a potential carcinogen which is one of the most overzealous and overly cautious lists of carcinogens compiled anywhere. People that work at our local DE mine in Lompoc do not suffer higher rates of cancer either. There are too many people spreading fear based on nonsense or the desire to profit. But I digress, yes, DE is a lung irritant as all fine abrasive material can be if too much is inhaled, but even guys working in a DE mine have little problems with it and they work there for decades...far longer than a chicken's life.
As far as its effectiveness on mites? Well recently, I've had a very sick hen with CRD infection that is so lethargic she has not been able to dirt bathe for two weeks. A few days ago I noticed a tiny light greenish/tan colored spider looking things running around on her face and comb. I dismissed it as just a tiny bug, but today I looked again with my glasses on and saw 10-12 of the little things running around on her face and comb and realized I had a problem. I took some DE which I had laying around and sucked it up into an ear syringe, shook it so the dust would be suspended in air and puffed a fine cloud of DE dust around her head when her eyes were closed (sleeping), her bum, under her wings and any other place I could access. I went back and inspected her comb 15 min later and found four dead mites and one still alive but struggling. You just can't beat 1st person empirical evidence like that to cut through the anti-DE fear mongering nonsense.