Hi animad, I agree with the other ABers about a vet trip being necessary but thought you might like to know that I too have a cat with contact dermatitis from the spot on flea treatments. I think you've done exactly the right thing by switching to a different dosing method, but the vet will probably cure the remaining irritation with a steroid injection. Don't worry about it being steroids, a one off is very safe. I have an Oriental tabby who has pretty constant excema & has to have fairly regular steroid injections to stay scab free & comfy. We manage with as few as possible & so far, so good for the last 5 years. She too is quite small but not as little as your girly.
One thing I do, that you may like to try, is to put a small cotton sock, child's size 12-3 with the toe chopped off, around cats neck, a bit like a leg-warmer for the neck! lol . It seems to protect the skin because it's loose & moves as they scratch but 'cos it's cotton, allows the skin to heal. You will probably need to use a smaller child's sock & of course you will need to keep her in whilst she is wearing it because it could be hooked up on something outside. Don't forget to check that it's clean & change to launder regularly. It may help until you can schedule a vet trip.
Wheezy cat at 10 yo is already considered an OAP in cat terms & she probably isn't doing such a good job grooming herself any more. A problem with furballs starting like this is most likely the first sign that she needs some help with her daily ablutions especially in the spring & autumn when coat changes take place - a gentle comb every day will do a lot to make her much more comfortable. Adding a teaspoon of oil to wheezy cat's dinner daily will help the transition of excess fur. But a vet's visit cannot be avoided because it can also be a sign of the beginning of age related dental problems so the vet can check & advise. Good luck.