I am sorry to hear about this concern.
The itching part of it is what is suspicious.
It almost sounds like a hypersensitivity reaction or allergy. So, I wouldn't be against you giving some antihistamine to see if helpful.
Benadryl can be given at a dose of 1mg per pound of body weight. Keep in mind, Benadryl tablets and liquids come in different sizes. So, an approximately 25 pound dog can get a full 25mg tablet or a half of a 50mg tablet. I usually avoid the liquid Benadryl in larger dogs (you would have to give too much of it). You can give Benadryl every 8-12 hours.
I would continue the essential fatty acid supplement.
Lastly, scabies (the contagious type of mange) can be very difficult to get on skin scrapings. So, a skin scraping, absolutely, needs to be repeated.
You also might want to consider getting a dermatologist involved.
You could even refer yourself to one by using this link to find one near you.
http://www.acvd.org/ & then select "find dermatologist"