Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am sorry to hear that your kitty has developed areas of dandruff (dry, flaky skin).
Will she will allow you to brush those areas out? If so that can help remove excess skin and hair and distribute coat oils, which should help at least somewhat.
Is she matted in those areas?
Truthfully once we get to the point of matting, especially if your cat that doesn't allow grooming, then shaving her down, combing out her undercoat and starting over growing in her coat and trying to keep up with brushing is best. If we just try to remove just the matted areas she will look very patchy and that doesn't solve our trouble with a matted, thick undercoat which will continue to create matts and areas of trapped, shed skin cells and oils as it grows out. She can look nice now and as her coat grows out with a "lion cut" that saves the coat on the legs, face and tail, but removes the matted and areas with excess dander on her body.
If this were just a few matts then sedating her and using a dry shampoos sometimes helps to break down oils and loosen the hair and skin clumps, and then shaving the few trouble areas is another alternative if there are just a few matts.
My concern though is why she has developed so much excess dead skin this year when she hasn't in the past.
There is a type of skin mite that causes increased skin scale, itchy skin, and secondary matting. If her skin seems very scaly, especially along the backline then this may be related to a skin parasite called Cheyletiella. This highly contagious mite can easily be picked up by coming in contact with an infected animal (dogs, cats and bunnies are the most likely carriers but guinea pigs are possible too) or the environment they were in. If she is a kitty that wanders out of the yard or you had strays or bunnies in your yard it would be more likely. These kitties tend to be very itchy so owners often notice more grooming too.
Another concern, especially if she is overweight, is a case of seborrhea and poor grooming due to difficulty reaching those areas, usually because of a stiffer spine (due to age) and being overweight contributes as it makes it difficult for her to reach everywhere to groom. When oil in the hair isn't distributed by normal licking/grooming the skin cells and hair get stuck and can matt a little, which pulls on the underlying skin making it painful. When we try to help by brushing that pulls even more and it hurts. If she has a little spinal arthritis that makes trying to groom those areas even more uncomfortable.
Long term to prevent matting I recommend an omega 3 fatty acid both for skin health and as a natural anti-inflammatory to improve joint health. I recommend an omega 3 fatty acid 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 a 10 pound cat could take 200mgs of EPA per day. A glucosamine/chondroitin supplement(like Dasuquin or Cosequin) may help too to ease joint pain. Together with the fatty acid they work synergistically, better then either one alone.
Finally if her food and water consumption have changed and she has had weight loss or gain she may be sick. Hyperthyroidism is one disease process we see quite a bit in cats. They tend to eat really well but maintain or even lose weight and have a very rapid skin cell and hair turnover so may matt more and may have problem skin and coats. They also tend to be more irritable and less amenable to your grooming help.
Ideally a veterinary visit and some blood testing to make sure all is well internally should be done. Your veterinarian can prescribe pain medication if they believe that her matting is related to arthritis pain.
Please let me know if you have any further questions.