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No worries, it took me a bit to realize my lowest girl wasn't shaggy but was in fact broken feathers from being picked on. I took your photos and circled some examples of broken feathers. From what I've seen in our flock, a way the girls communicate is to either peck at the head or peck at the back. Back pecking seems to be more related to pulling and when it happens the feathers actually break off so you get the v-shapes at the end of the feather. Pullet feathers should usually be rounded so the fact that these girls are missing a bunch of their outer layer and they appear broken it makes me think either there's a lot more squabbling going on than you have seen or they are pecking at the feathers themselves due to some irritant.
I haven't seen mites in person, just read some articles. Could also be lice not mites and the black spots could be droppings. Lice tend to go further up into the feathers and not live down at the surface, only coming down for feeding time. If I recall, there are some mites that only come out at night and tend to stay on the roosts during the day. So you could see dried blood as black spots (similar to with fleas in cats/dogs. I've read mites and lice tend to be around the vent and under the wings.
Missing neck feathers... do you have any roos by chance?