Issue with my naked-butt chicken

As you well know if you life in the northern part of the US, this last winter was rough. We had a large stretch where it was -20 degrees plus. During that time, things were tough. I confined by birds to the inner coop for their safety, and brought them out hot water ever couple of hours.

During this time, while everyone was inside and stressed, two of my chickens somehow got a poop balls on the fluffy feathers on their butts. I couldn't get them what to bathe them, so I tried working them with my fingers and breaking them off a bit. Eventually, two of my birds had feathers rip off their butts with poop balls. I used hen healer paste on them to cover their butts the rest of the winter, but this barred rock never grew those feathers back. Her butt is also somewhat chapped looking. My fear is that she got slight frostbite on her butt.

Since then, she's been getting picked on. I put a saddle coat on her to cover her body, but they would pick at her bare butt. Today, I saw some blood near her vent, so I quickly removed her, cleaned the slight wound, sprayed her with silver spray, and rubbed the area with hen healer.

Her poop is normal, she's active, eating and drinking. I checked her for parasites and found none. I saw no maggots or anything to make me think fly strike either. I think she has lost some weight, but I think it's because she's been broody and being bullied. I'm going to keep her in this kennel until she heals up a bit.

Has anyone dealt with the bare butts? Why did only some of the feathers grow back since January? Will she be bare forever? How can I help her grow her feathers back so they stop picking on her?