I was just reading your post and see that your cat is no longer eating. The Vitamin C you could dissolve in water and give her that through a syringe. It should boost her immune system quickly.
My understanding is that it takes us six days to see when a companion is ready to cross over, just as it takes six days for a supplement to show any effect. The situation quite honestly sounds dire.
I can only imagine how hard this is for you, but if she is in pain, please let her cross over.