I am sure other people here will offer ideas.

Since your vet has ruled out parasites, and food allergies usually show themselves in inflammation around the ears, I wonder if this is from stress and has become behavioral? Like a hot spot that becomes worse and habitual due to incessant licking?

Would your vet consider a topical antiinflammatory with an e-collar, to prevent ingestion due to licking? But I do not know if topical steroids are appropriate for cats, certainly not appropriate for ingesting!

I am interested in other's thoughts here.

One of our cats is very attached to my DH. When we'd returned from a family visit out-of-state, we noticed that she had missing patches of fur (about an inch long) on the undersides of both front legs. When a biopsy showed nothing, and cortisone injections really didn't help, and it got worse (tops of front paws, back legs, more missing fur from front legs), we pretty much felt that it was likely OCD behavior/separation anxiety.

We took her to a dermatology vet who agreed that our theory was pretty likely and started her on Elavil (amitriptyline). It's an antidepressant (for the OCD/sep anxiety) that also tends to have anti-itch properties, is generally well-tolerated by cats, with no side-effects (that we can tell), and the derm vet feels that there are no contraindications for long-term/lifetime usage.

It helped eliminate the "overgrooming," and our cat will likely need to remain on it for life (attempts to reduce her dosing schedule down to every 3rd day weren't successful, so we bumped her back up to daily dosing, and will go back to every other day in a week or so and see if she's ok on that).

We use a transdermal gel, and rub it inside her ear. Two hints if that's what they try on your cat next: we had the compounding pharmacy (Belvidere Pharmacy in NJ) compound it in a more concentrated formula so we can use less cream for the same medicine dosage (it's thicker and it takes less time to rub it in than the more watery and larger am't of the "regular" concentration). Also, I try not to go too many days before wiping out her ears with a wet piece of paper towel or gauze (the cream builds/cakes up over time). She actually likes when I do that--maybe she thinks I'm grooming her!

Good luck!

Thank you so much for your suggestions! This month, Daisy is taking her time to start the licking herself, as she only lasts 3 weeks after the shot...
The only things that have changed...I dewormed her again with Drontal and I am fostering little kittens...She used to be the super foster mom and would clean and even nurse the kittens I fostered
We will see if she starts licking herself again - I hope not!