@aart "That stuff" I was referring to was mentioned by Michaels1715 in the post just above my post: Garden and Poultry dust. I assumed Michaels1715 knew what I was referring to, but I apologize for not explaining thoroughly for everyone else.
@Michaels1715 That stuff really is toxic to chickens, and in my flock the mites became immune to it. I use neem seed oil now, which is messy on the feathers, but works and is not toxic to chickens. I only have to treat the chickens that sit around a lot, or broody, and I had a rooster who...
That does look like rooster damage on the back, because there is not always feather damage on the shoulder/wing area. The feather loss to the bottom is not from a rooster, and I agree that her abdomen looks swollen and needs to be checked out.