you might want to wash her butt more often than just once a day - if she's getting quick build up or old build up, then she needs more cleaning. Gently of course.
If the problem is mites, then you will want to ensure you have thoroughly treated the coop area. Also, since you are concerned about the safety of the eggs for consumption, have you considered using an organic solution? One of the best explanations and instructions I've seen in one place is this...