You did a really good job getting these pictures! Really a guessing game at this point, but you can do things that will do no harm and may do some good for her, especially give the fact her weight seems low compared to the others the same breed and age. If you can get a fecal float done to see if there are internal parasites I would do that, but I know a lot of vets don't offer the service for chickens, so if that isn't an option, consider treating for internal parasites. I've seen several options for doing that, and you can probably find them all here at BYC, but my default is always to go with whatever product(s) and dosage(s) recommended by
@casportpony. I, personally, find using pour-on ivermectin, 4-6 drops at the base of the neck (depending on the weight/size of the bird - 4 for bantams, 5-6 for most large fowl has worked for me), and repeat in 7-10 days, has been low stress for both birds and me, and seems to treat a plethora of parasites. You could also offer her some cooked egg and/or plain yogurt as a healthy treat which may boost her general health, and vitamins and/or electrolytes in her water in case she is low on something or just needs the extra help.