Sounds like they are lacking protein in their diet. Give them gamebird feed to eat, that should stop the feather picking. Then wean them off the gamebird feed after about a month back to layer feed.
Sevin dust IS less toxic than ivermectin, and much safer to use against lice/mites. Sevin dust is much cheaper than ivermectin as well. Besides, if it is lice/mites, you have to treat the inside of the coop(s) as well....cant do that with ivermectin.