My name is***** and I am sorry to hear that Isabella is so itchy.
It sounds like you have tried several different things, but I have a few more ideas for you to try:
-has your pup been skin scraped for sarcoptic mange? This is a very tricky parasite to find, but if the dog has it, it will make them EXTREMELY itchy. The treatment is Revolution drops on the neck or Ivermectin. These both can be given without finding the mite as a "response to treatment" trial. Both of these are extremely safe so there is no risk in trying this.
-have you tried a hydrolyzed protein diet? This is where the protein molecules are predigested prior to putting them in the kibble so there are no large molecules for the immune system to react to. While a grain free diet sounds good on TV, grains are actually very rarely the culprit for allergens and it is usually the proteins that are the problem. Royal Canin, Purina and Science diet all make hydrolyzed protein diets.
-have you tried using dog shampoo such as a soap free shampoo? Human products are very drying to dog skin and not pH balanced for dog skin so they can throw the bacteria out-of-whack on the skin and make the itchiness worse.
-lastly, do you have good flea control on all the animals in the house, not just the dogs? Dogs with flea allergies will react with only one flea bite and become very itchy.
Does this help? Do you have any more questions?