I'm really sorry to hear about your girl's excessive grooming and scratching.
The short answer is that there are lots of reasons for a cat to be itchy. Sometimes it ends up being less expensive to run diagnostic testing and figure out why she is itchy in the first place. The other option is to try treating her for the more common reasons for itchy skin/excessive grooming and see how she responds.
The most common reason for itchy skin is flea allergy dermatitis. Flea allergy dermatitis is horrible. If you don't see any fleas you might not have a huge population. And in allergic cats there doesn't need to be many, it only takes one bite a month for an allergic cat to itch like crazy. If you have other pets who aren't itchy they may not be allergic. Ideally I recommend topical flea products be applied every 3 weeks during the summer/fall months in an allergic cat when fleas are at their peak, or with a active problem and then monthly as a prevention. Many over the counter products have a problem with fleas being resistant and some can be down right dangerous. I never recommend any products that Hartz or Sargent's make. I recommend Advantage II or Frontline Plus, or the new Seresto Flea collar. I don't normally recommend flea collars, but this one really works and lasts for at least 6-8 months. All pets in the house must be treated as if you don't the nonallergic ones serve as a flea reservoir for the allergic one and you'll never solve your problem.
Inhaled allergies (to pollens, dust mites or mold spores) are another possibility for itchy skin.
To control the symptoms of allergic dermatitis you can try giving her a combination of an antihistamine and an omega 3 fatty acid supplement. Antihistamines to try include:
1) Benadryl (diphenhydramine only, the combination products with decongestants and acetaminophen are toxic to cats) at a dose of 1mg to 2mg per pound of body weight every 8 hours. That's a half of 25mg tablet every 8 hours for a cat that is 8 to 15 pounds. Benadryl is very bitter and some cats will drool excessively or may even vomit because of that. That doesn't mean it is making her ill, she just hates the taste. If that's the case with her try a different antihistamine.
2) Chlorpheniramine at 4mg twice daily.
3) Zyrtec (Cetirizine hydrochloride) at a dose of 5 mg per cat given orally every 24 hours. Make sure it is NOT the formulation with a decongestant (such as Zyrtec-D) because cats cannot tolerate decongestants.
Combined with high doses of omega-3 fatty acids antihistamines work to relieve her itchiness. I like 3V caps or Derm Caps as they are reputable, high quality products. I recommend a dose based upon the EPA portion (eicosapentanoic acid) of the supplement as if we do that the rest of the supplement will be properly balanced. Give her 20mg of EPA per pound of body weight per day. For example an 8 pound cat could take 160mg of EPA per day. Antihistamines and omega-3 fatty acids work synergistically, better together than either one alone. These should help reduce the itch. Be aware that antihistamines can cause drowsiness or hyperactivity which should resolve with continued use.
Food allergy is another reason for excess grooming. You might wish to try a hypoallergenic food for 8 weeks or so. My suggestions are Hills Z/D or Royal Canin Duck and Green peas. She will need to eat only that food, no other treats or foods during her food trial.
Over the counter foods may be labeled hypoallergenic but they are unlikely to truly be so.
The trouble with "limited ingredient", "hypoallergenic" or "low allergy" pet store brands is that the same machinery is used on multiple lots of food without sterilization cleaning in between. So for example even if a food says it has salmon and rice if the previous batch had beef and corn then you will get traces of those ingredients in your bag of food. Not a big deal if your kitty isn't allergic to those ingredients but a waste of money thinking that the food was hypoallergenic, and not good for her if those happen to be allergens for your kitty.
The veterinary brand true hypoallergenic foods are more expensive because it isn't cheap to thoroughly remove all traces of a previous food mixture from the machines used to process food or to hydrolyze the proteins in the food. And the only ingredients in that food, even at a trace level, are what is listed on the bag.
If there are other cats in the home that she doesn't get along with or outdoors that upset her perhaps this is stress related. If so a product called Feliway, which is a synthetic version of a calming feline pheromone, may help. You can purchase this from petstores or online.
If she goes outside other parasites like Cheyletiella, demodex, or sarcoptic mange are possible causes as well.
For now I would start with continued flea control, an antihistamine and an omega 3 fatty acid supplement. If she's not significantly better in 2 weeks then it's time for a veterinary examination to look further for problems and/or a hypoallergenic food trial.
Please let me know if you have further questions by replying to this post.