Sevin is carbaryl, no longer approved for use on chickens. Don't buy it! Permethrin comes as a dust or spray concentrate, which is approved and much easier to use. As you found out , DE is useless!
For anything, including coop cleaning, get N95 face masks, available in the paint department at the big box stores. Nothing less effective will protect you!
Permethrin dust is usually sold as 'Poultry dust' and will work but is harder to use. Permethrin spray concentrate is inexpensive easy to use, and very effective. It's not the best choice in extremely cold weather.
Go out at night with a flashlight, dust or spray, and hopefully a helper. For dusting, pick up each bird off the roost and treat. For spray, just spray them right on their roost!
Mary