It sounds like Charlotte's really had some difficult issues, Melody. We're sure that, if she could, she'd thank you for all your care and concern!
It's our guess that she has chronic rhinosinusitis, though an examination and round of tests is necessary to diagnose it accurately. If you'd like to know more, we recommend a very good overview of current thinking on the topic. It's lengthy, and filled with veterinary jargon, but well worth a thorough read:
The short version is that they recommend an extended course of antibiotics. If that fails to resolve the issue medically, surgery is suggested.
We recommend against using traditional antibiotics to treat an infection except as a last resort because of the way it wreaks havoc with a cat's digestive tract. Garlic can often be a good substitute at the beginning of an infection.
It might be a good idea to start adding garlic to her meals if she'll accept it. You can do this by mixing 1 tablespoon of distilled or spring water with 1/8 teaspoon of tamari soy sauce and 1/4 teaspoon of raw garlic crushed in a garlic press. Let the mixture stand for thrtee hours and store it in the refrigerator. It will last for three days that way. Then mix 1/16 teaspoon into each of Charlotte's meals. This can be done in perpetuity and will help her to fight off infections while also serving as an antiparasitic. It also lowers blood sugar. There really is no down side to adding garlic to her meals starting ASAP.
It can also help to give her normal saline solution eye and nose drops before each meal. This will help to sooth and clear her nose, sinuses, and throat. Never use solutions made for humans. It's best to make the normal saline solution yourself by dissolving 1/8 teaspoon of sea salt into 1/2 cup of boiled water. Cool it thoroughly and store it covered in the refrigerator.
Before each use, warm it to body temperature and load it into a dropper. Place Charlotte on a tabletop or counter and massage her face. Hold her head from behind and administer three drops of the solution to the inner corner of each eye. Massage the skin above and below her eyes to get as much of the solution in the eyes as possible. Be sure to talk her through the procedure, telling her how you're doing this to help and comfort her. She may not understand the words, but your firm manner and comforting tone will go a long way to putting her at ease. Blot any excess liquid and release her head. If she flicks her tongue out, you've succeeded and the solution has run down her throat.
The procedure for nose drops is pretty much the same, but you may need to hold her bead back for five or six seconds to make sure the solution has gotten in deep enough to do some good. You can also administer several rounds of nose drops because Chralotte will find that they actually feel very soothing. We recommend three rounds of three drops per nostril. Be sure to have some soft tissues handy as Chralotte may sneeze out congestion shortly after this treatment.
We always recommend getting to the root cause first in order to alleviate subsequent infections. To do so, you'll have to find a veterinarian who's willing to dig into Charlotte's case and work with you to get to the bottom of her issues. These days, so many doctors, both human and veterinary, want to jump to an easily treatable conclusion. You may benefit from scheduling a brief consultation session with any potential vets before you even bring Charlotte in. If you can find a feline-only vet, you're halfway there. You'll know when you have the right one. If you have any other active cat rescues you associate with, you may want to ask them who they use.
A veterinary homeopath is not out of the question, but you have to be careful to be sure you have one who is of the more practical variety. It's our hope that one day we'll all be treated by those who are well versed in both traditional "western" medicine and homeopathic treatments. Until then, it's wise to be cautious.
We wish you and Charlotte all the best, Melody! Good luck!