Thank you for the new pictures that provide more insight.
The aviary appears rather bare, some shrubs would provide shade and resting areas for your chickens.
The additional DE and sulfur in their dust bath will dry out the skin. So maybe refrain from adding too much too often.
Welcome to the BYC community @Elspeth Dinsmore :frow
After reading your story and looking at the pictures I think there might be some feather picking going on in your flock.
How much protein does your feed provide? Could you post some more pictures of your run? Aviary and run seem to be rather...