You can dust chickens for parasites at any time, it won't hurt.Our girls haven't laid an egg in well over a month and there have been feathers everywhere. We've had chickens for several years and have never seen such a protracted molt. Their combs are pale and they don't seem to want to eat much (except sunflower seeds but we know their layer feed is better for them.) Does it hurt to treat them for mites even if they might not have them? Can you eat the eggs of the chickens you are treating?
If there are feathers everywhere than they are probably molting. Stop or cut down on the sunflower seeds. They will cause deadly internal fat if fed too much. Switch your birds off the layer and onto a higher protein feed like an all flock or non medicated grower. A separate bowl of oyster shells will take care of any extra calcium needs. Birds will get deficient on a layer year round, especially during the molt which requires a higher protein. If they aren't receiving enough protein it will take much longer to molt.
Comes shrink up and turn pale to reflect a bird is not in a reproductive condition. It's normal whenever a hen stops laying, especially during the molt. It also happens when they are unwell, so make sure they are eating and drinking and otherwise mostly fine. Some birds will look sick during the molt too.