I'm sorry your kitty's not doing so well. I'm not an expert of skin diseases in cats and I don't have much experience in that area, but I do have a couple of questions for you, if you don't mind. When did the itching start? Did it coincide with a change in diet, living arrangements, or weather? Has it ever seemed to get better -- even if only for a short period of time? Is the itching widespread or limited to one area of your cat's body? What has been ruled out by the vet? And, if allergies are being suspected, how long has it been since you have switched foods and what methods are you using to diagnose the presence of allergies? I only ask because I went through a very similar experience with my dog this past year.
That all being said, as far as E-collars are concerned, I used an inflatable one for 6 months on my toy fox terrier. I only recently saw the same collar available for cats and while it was an excellent investment for my dog, I don't know how it would translate for a cat. The inflatable E-collar allowed my dog to increase her field of vision from that which is offered with the traditional "cone." It was also a lot more comfortable for her and she was able to continue to eat and play normally without the restrictions the larger collars are known for. However, it had its downsides as well. It is inflatable and therefore runs the risk of being punctured and losing its ability to hold air. This only happened once after my dogs were playing particularly rough with one another, but it is possible. Thankfully there are strips and products available to repair small punctures and I never needed to replace the original collar, but it was frustrating at the time and could potentially be an issue for you -- especially since you're dealing with a cat. Also, while the inflatable collar allows for more comfort and easier movement, it doesn't provide as much protection. It's a lot smaller than the traditional collars and is a different shape -- where the E-collars are cone-shaped, the inflatable collar is shaped more like a wide doughnut and sits on their neck as opposed to coming up over their face and head. In the end, it's more about what your cat needs as far as protection as well as what he or she will tolerate, what you're looking for based on vet recommendations, and your budget. What worked for me and my dog may not be appropriate for you and your cat, but at least it's another option that can be considered -- especially if you're needing a more long-term solution.
I would also suggest potentially looking into seeing another vet for a second opinion. Vets are human and are fallable as much as we are and there is always the possibility that your vet missed something or is not seeing the situation clearly. Holistic and alternative therapies, in conjunction with western medicine, are also additional options to look into. I went through a lot with my dog and it was deeply concerning for quite a long time. I can empathize with you on the frustration, overwhelming sense of powerlessness, fear, and worry that's present when one of our loved ones is not doing well, but please know that there are other options and that sometimes it takes advocacy on our part to get the answers we (owners and pets alike) need. Thankfully, I can report that my dog's situation has improved dramatically and that she's nearing a 100% recovery, but it took a lot to get to this place and if nothing else I want to express that there is an answer out there as to what is causing your cat to itch so severely -- it may take time, patience, and even pushing the limits, but please don't give up. Because, if you do (and I realize that you're not, this is merely hypothetical) you do a great disservice to both yourself and your furry loved one.
Good luck. I hope the search for answers brings both you and your kitty hope, relief, and health.