There's a reason the government is banning the use of Sevin (& its a good one) - its a known carcinogen. When you put that stuff on your chickens it gets on you, into the soil and into the ground water!
When my flock had mites I power washed the inside of my coop, bathed the hens in a bucket with dish soap, vaselined their legs (at least three times/wk), sprayed the inside of the coop (roost bars, nest boxes and shavings) with a spray bottle containing water, crushed garlic and neem oil. I also dusted the inside of the coop with DE and a small amount of cedar sawdust. It does work but you have to be consistent. The problem is chickens get mites from wild birds so if you have backyard hens or free- ranging birds you may also get mites.
We need to start looking for alternatives to highly toxic insecticides whose impact is more far-reaching than just killing mites. The alternatives might take longer but they are healthier for all of us.