Things I would consider trying.
Douxo Calm Mousse - You can apply it every three days. Skip the days you give a bath with medicated shampoo.
Don't shy away from using the Benadryl it won't hurt her and if it makes her more comfortable then that is good. 1mg per 1 pound of dog 2-3 times a day. If she has side effects like being sleepy try another antihistamine like Claritin or Zytrec.
Boosting up her immune system:
Virgin Coconut Oil 1 tsp per 10 pounds of body weight. Once a day. You can feed it off of a spoon to her or put it with her food.
NuVet Plus - Canine Vitamin yes it is one the expensive side it is a high quality supplement. https://www.nuvetlabs.com/order_new/index.asp
Abbey is looking for a forever home
Thanks for the help