I had an Aussie with allergies. First off, mange is not that hard to treat in a young dog, so don't freak about that. I would work with a vet first to see what they find, blood tests etc. A lot of vets out there have keen interest in dermatology, so find one of them. Make sure the flea thing is taken care of and any internal parasites first. I don't think flea collars are that affective. I use advantix II. Then I would go to a limited ingredient diet (LID). Natural Balance has a lot of them. Pick a novel (never before had) protein and carb. There are probably other LIDs. A diet test takes 12 weeks, so don't expect instant results. Use the kibble as treats, no cheating. I actually had a skin test done on my dog and he had lots of environmental reactions. Bigger cities have vets who are dermatologists only. We did shots for a year with no luck and then antihistamines with a little luck. I just managed and it was okay. Actually got better as he aged and I lost him at 13 years old a week and a half ago. But that is my story. It becomes a bit of a mystery at times. Good luck and don't freak out.
Dutch Shepherd - Ptygo (Tee-Go) de las Flores
Rescue GSD - Freyja (Husband's Dog)