Viral infections, such as herpes virus and feline leukemia virus sometimes cause scabby itchy sores on the head or other areas of the body. Phemphigus foliaceus, an immune mediated disease, can cause itchiness around the face and head. There are probably other conditions that I can't think of right now. It is probably going to be necessary to do more diagnostic testing and possibly to try treating for some other conditions, in order to finally find a diagnosis and appropriate treatment. Fungal cultures for ringworm, using a toothbrush culture technique, can be helpful as an early test since it is easy to do and not very expensive.
Skin scraping for mites is a good idea, too, for the same reasons.
Once these easy tests are done, it gets a little harder to decide what to do next. Skin biopsies are helpful in many cases, referral to a veterinary dermatologist is a good choice, food trials to rule out food allergy might help and sometimes it is better to try a different treatment approach to see if that will help. Your vet can help you decide which direction to go in based on the progress of the disease. It really might be worth having your black and white cat examined by a veterinary dermatologist if your vet isn't able to find a problem with further examination. It might be just coincidence, but since she came home she scratches alot.