When you have chicks or young pullets not yet laying, it is good to feed only chick starter or all flock feed, so the youngsters do not get too much calcium. Layer has 4 times the calcium as chick starter or all flock, and can lead to kidney issues or gout. Crushed oyster shell is used in separate container for the layers to take as they need it.
I would use the dandruff shampoo as Wyorp Rock recommended suggested, but just a normal balanced chicken feed with perhaps a little egg for a treat. Corn or scratch will only dilute the good balance the feed has. Feather cysts from ingrown feathers can cause bump on skin, but the dermatitis on her back is concerning. The skin type of Mareks could be possible. Hopefully not. The comb may be from pecking or rubbing/damaging it on chicken wire. I would be tempted to give her 1/4 to 1/2 of a human super B complex vitamin tablet to see if that has an affect on her skin. Walmart has some Spring Valley brand for about $8. It has biotin and all of the other B vitamins. It can be given directly in the beak after she eats or crushed and given with food.