The scales/scabs sound like
scalp psoriasis. It can occur without any other skin involvement. Color can be slivery white to a sickly yellow.
Sebo-psoriasis is scalp psoriasis with scalp seborrhoeic dermatitis on top of it, the lesions tend to be more yellow and greasy in appearance. Do the lesions follow your hairline or behind your ears? Have you tried coal tar shampoos?
Scalp acne is the mildest form of scalp folliculitis, an inflammatory condition of the hair follicles.
In your case it sounds like you been through the ringer
trying everything, maybe it's time to see a Dermatologist for a firm diagnosis and Rx strength products. You don't want to wait till there's irreversible hair loss.