Trim the feathers when they are starting to have sight problems. I can't tell you an exact length of time, because I don't trim mine. For the mites, you should try using the wormer "Eprinex", an Ivomectin based pour on cattle wormer. It will kill worms and external parasites. It supposedly has no withdrawl time, but I hold off on eating the eggs for a week.
Since you don't want to use chemicals, you might try adding DE (diotamacious(sp?) earth). Use ONLY FOOD GRADE!!! Some feed stores readily carry it. Do NOT use the stuff for pool filters, ONLY Food Grade, and it will be marked as such on the bag. Rather expensive, and for me, even with a respiratory mask, leaves me coughing for days! Dust the coop with it, put it in the nest boxes under the bedding, and put some in their dust baths. I don't like it, and the 50# sack I have here is still 3/4 of the way full.
You might also try bathing the chickens with a Flea & Tick shampoo, after cleaning out the coop. You can use a flea & tick spray inside the coop, keeping the chooks out for a day while it airs out some. Depending on how much energy you have and how many chooks you need to bathe, this might be the best way for you? It won't do much of anything though if you don't thoroughly clean the coop and run!