Bad Teeth, diet, special care considerations?

[CatDefender]

I rescue cats. I have an elderly Siamese (Jixy), when I rescued her she was an adult and I've had her for 12 years so I estimate she is around 15 years old. She has bad teeth, she has had halitosis as long as I can remember. Now she has only a few good teeth left and she isn't eating dry food. She will eat some finely chopped wet food. I'm taking her to the vet soon for a checkup but I wonder, can a cat survive on a liquid diet if necessary? I feel guilty about the condition of her teeth but I have tried the brush her teeth as well as all my cats but none will sit still for it. I don't want her to have surgery because of her age. Anyway, I'm just worried about her and was wondering if anyone had gone through this experience, and any successful strategies you used to make sure that the cat's nutritional needs were met.