Yes that mounting behavior is a dominance thing. If you don’t have a dominant rooster in the flock, the dominant hen will sometimes mount other hens, usually younger pullets, to show her dominance. That part is normal. By them squatting for her, they accept her dominance.
I’m not sure what causes that baldness. Some people will immediately blame your rooster but since you don’t have one I doubt that is it. That dominant hen is probably not mounting them enough to cause that, especially on herself. You say you are using powder, but have you checked for roost mites. These only come out at night and will run from light. You need to check the vent area fairly quickly by flashlight after dark to see if you see anything scurrying away from the light.
Some hens just have brittle feathers. They break really easily. If that is the problem, it’s possible something about their lifestyle causes their feathers in those specific areas to break. Maybe something as simple as dust-bathing though that would be really brittle.
I’m not sure that chickens don’t shed some when the weather turns warm either, like a dog shedding its winter coat. It could be that the shedding is not as controlled in some chickens as it should be. That’s just a guess of mine, I have nothing to back that up.
The feather picking bothers me some, especially since it appears it is only one hen doing it. If it were several hens I’d suggest upping the protein content of their feed, but them grooming one another isn’t that unusual. I saw a hen pick a bald spot on a rooster’s neck just grooming him. He stood there and acted like he enjoyed it. It’s not at all unusual for chickens to eat small feathers floating around. That’s just free protein as feathers are mostly protein. The way I read this it is just the dominant hen doing this. That sounds like it is some form of aggression. I agree to separate her for a week or so to try to bring her down in the pecking order. If that doesn’t work you may find that your flock is so much more peaceful without her that you might make a change. Good luck with that.
Then again, a lot of this may just be due to them resorting the pecking order. It sounds like you just added the two pullets and that they are still pretty young. They will be at the bottom of the pecking order until they mature enough to force their own place in it. In their case a lot of pecking order ranking is based on maturity.
When integrating younger chickens I’ve seen a hen, always for me a very low ranking mature hen, leave her normal roosting spot to go harass younger lower ranking chickens. It’s as if she is jealous and wants to maintain what little social status she has. I’ve had chicks leave the roost to find a safer place to sleep so often that I finally installed a roost especially for them, lower than the main roosts, higher than the nests, and separated horizontally so they could safely sleep without being harassed. I have eight 17 week old chicks using it right now. Before I installed that roost, they would sometimes move from the main roost to the nests but I have had chicks leave the coop itself to roost outside to get away from that bully.
The only thing that concerns me at all in all you have said is that hen picking feathers to eat them. That might be some form of aggression that could lead to much worse if she does cause bleeding or a raw spot. Chickens can become cannibalistic with blood or an open wound. I think the first step for me would be to isolate her for a week or so to try to modify her behavior. See if that works. If he were just eating loose feathers floating around, I would not be concerned at all. It’s the picking that bothers me.
Good luck with it.
