If you can get your cat to take the CS without traumatizing her too much, that would be good. I wouldn't try to force feed her with too much food, even baby food if she really doesn't want to eat. It is possible that she is hanging on this long for you because she is worried about you, more than for herself. Animals do not fear going to the other side like humans do. When they get to suffering too much, they are ready to go and they know it. If you can make her as comfortable as possible without trauma, I would see if she can tell you what she really wants to do in this situation. Because it would be so stressful to you to be with her if you do choose to have her put to sleep, it might be best if you weren't there at that very moment - again for you only. Cats like to cross over without their pet parents many times because they know it hurts us too much - all my opinion anyway. In the meantime send her as much love from you as possible and if you need to let her go, if you can let her go if she needs to...that would help her.
If she comes around with the CS, is it possible for you to seek the help of an alternative practicing vet? Alternative options can work, but usually need the guidance of a good alternative vet.
I wouldn't try the Watermelon
juice option, as cats have a different system than we do being obligate carnivores and their digestive system is very acidic. All cat and dog health really starts with real food and it really should be as close to a natural raw food diet as possible. Plus to not vaccinate the heck out of them, especially cats. It is proven that cats can develop cancer from vaccinations. If you work with an holistic alternative vet, they can guide you with this and other alternatives to vaccines. Plus they can help you with your heartworm challenge.
I have also used an animal communicator and energy healer for my cat who was hit by a car and it worked wonders. This is just another idea for you to consider.
I hope you and your kitty are going to be OK. I am sending my thoughts and prayers to you both.