Oh, butterchaps. I see you. You're out there with a spray bottle full of Electro PSP, a bag of diatomaceous earth, and a rooster who has decided that his purpose in life is to be a tiny, feathered tyrant. You've got nineteen hens and one very naked bully-boy, and you're staring at a flock that looks less like a happy backyard flock and more like a group of plucked turkeys who lost a bar fight. I respect the effort. I also fear for your sanity.
Let's start with the rooster. You say he's been chasing the hens, mounting them for a few seconds, and then just standing there like a confused goon. That is not breeding, friend. That is a power trip. Your rooster has decided that he is the king of a bare-backed kingdom, and he's taking his job very seriously. The feather loss on his back is probably not from mites at this point, it's from his own hens. When a rooster mounts repeatedly, his toenails and the friction of his body scrape feathers right off the ladies. And if he's bullying them, they're stressed, and stressed chickens don't grow feathers well. You treated for mites, which was smart, but if the feather loss started slow and then got worse around the time he turned into a jerk, the real culprit might be wearing spurs.
As for the rest of the flock, the bare bellies and the tail regions, that points to a couple of possibilities. One, your treatment may have knocked down the mite population but not eradicated it. Electro PSP is good stuff, but one spray-and-done while you're dealing with an established infestation is like telling a flood to stop by waving at it. Two, you mentioned seeing little starts of new feathers that never go anywhere. That is a classic sign of ongoing irritation. Whether it's mites, feather picking, or the rooster's daily ambush routine, those pin feathers get broken off before they can grow. Diatomaceous earth is fine as a dust bath additive, but it's not a cure-all. If the mites are still hanging out in the cracks of the coop or on the birds themselves, you need a second treatment in about 10 to 14 days to catch the next generation.
What would I tell you if you knocked on my virtual door? Treat again. Hit the coop, hit the roosts, hit every bird, and do it twice, three weeks apart. Give your rooster a break, or give him a time-out in a separate pen for a week to let the hens recover without his "attention." And if the new feathers still refuse to grow? Yes, it may be a waiting game until their next molt. But don't count on molt to fix a bullying problem. Separate him for a week, treat again, and see if the ladies start looking less like refugees. If they do, you'll know the real problem was wearing feathers all along.