I don't know at this point. I'm out of ideas. The reasons for pecking each other's feathers are usually:
1) diet - lack of protein, sodium or other nutritional essentials. You've covered that with the change of feed and restricting treats so you know they're getting enough of the good stuff.
2) too small a space -- From what you said in earlier posts, they have plenty of room and have more than enough personal space.
3) stress -- you'd have to decide by their behavior if you feel they are stressed and what might be causing the stress.
4) boredom -- if they don't have enough stimulation in their environment, they can peck each other out of boredom.
5) visible blood veins -- when chickens recover from a molt, the new feather shafts have a blood vein in them that are visible to other chickens. They'll peck at that blood vein. You didn't mention molting, so that's probably not it.
6) not enough nest boxes -- they could peck at each other if a hen is in the nestbox and another hen wants that box. They can get pretty snitty if their favorite box isn't available.
The more I think about it, the more I think that external parasites would cause the chickens to peck at themselves, not each other. One thing I don't remember asking, (and if you mentioned it, I forgot) is where are they pecking each other? Are they pecking on the head and neck in an aggressive act or is it more the back, the tail and wings? Are they pecking in an effort to establish or challenge the 'pecking order' of dominance and subsequently eating the feathers that they pulled out?
I found this article on feather pecking, which may or may not be of any help:
https://en.wikipedia.org/wiki/Feather_pecking