I did not read all of the posts, but I looked at the pics and many of your questions.
My advice would be to pluck the feathers, They are dead and anything dead around an injury can help bring bacteria flocking to the area.
Also, dead feathers can hold bacteria inside the feather shaft AND the hole in the flesh that holds it.
Plucking may hurt for a bit, the same as pulling scabs off hurt them, but is just as necessary to drain infection from the area.
You may want to bandage the area to reduce the incidence of flies. One a birds' wing gets infested with those amputation becomes necessary. The maggots crawl into the wing and eat any dead flesh/ infected masses and leave holes for new infection to follow...all in places you cannot see.
Flush well and frequently with saline, you can get this at most drug stores.
Some of the feathers will need plucked with tweezers. Do not leave the shaft in. If the shaft is left in, it can give a direct path for pathogens to reach the blood supply.
I have had to deal with this in several rescue birds with injuries. It seems to apply universally about plucking feathers away from damaged and injured areas.
I hope your chicken recovers well!