Sorry about how things have turned out. Mites can really sap their energy and make them anemic. Do you know if the breeder vaccinates their chicks for Mareks?
Are you certain they are mites and not lice? Permethrin garden dust or permethrin spray, available at most feed stores to treat the chicken, can be applied at 7 day intervals to kill the lice or mites. I would return the chicken to the breeder, no questions asked if possible. No one should have...