Mitch, first of all, thank you for the detailed information. That is extremely helpful and allows me to help you so much better! Secondly, good for you for your patience and dilligence in addressing this issue for your dog. It is so frustrating to see your pet itchy and uncomfortable, and if the tags are constantly clanking together, annoying too, right!?
So, the food trial is a good one and important. It can take up to 12 weeks to see a marked improvment if the problem is due to food allergies. However, it sounds more like your dog has inhalant allergies (atopy) like hayfever that we can get. If that is the case, changing food will not make a difference. Since you have been at this for so long, I strongly suggest you discuss with your veterinarian a referral to an veterinary dermatologist. Yes, we have allergy/dermatology specialists for pets, and THEY KNOW ITCHING! There are many medications that can be more effective than benadryl for some dogs and allergy treatments may be warranted. I promise, you will not be disappointed if you go to one.
You can go on line to find a specialist near you if your vet cannot recommend one.
The American College of Veterinary Dermatology certifies veterinarians who have expertise and specialized training in the diagnosis and treatment of animals with benign and malignant disorders of the skin, mouth, hair ears and nails. A veterinary dermatologist also has had significant training and is experienced in the treatment of allergic disorders.