While sugar has its uses, if she has an infection it's best to avoid white sugar. Raw sugar, or honey, would do better there.
If she wasn't brooding, then she hasn't starved herself. Even when they stop eating so much when broody, it's because they don't need it, not because they're denying themselves, except in the cases of those hens lacking complete instinct.
Some causes of starvation include crop or gut blockage, leucosis, organ failure, poisoning/toxicity, and of course any damage to the intestinal lining which could prevent her absorbing food. There are many causes of an animal starving to death while still eating normally. They can be eating normally and the poops can be looking healthy and coming out the other end normally right up until they drop dead from starvation, with some gut disorders.
Clarify a few things,you posted she had diarrhea,but was only pooping 2-3 times. You mentioned in above post that rooster had pecked her,is there a wound? You mentioned she had a black comb,explain.
Yes,broody hens just sit and poop only a couple of times,huge poops. They do not eat properly. Does she growl or make funny noises when you approach her?
So there is no wound,just feathers missing? Is it cold where you are,this might explain black tips on comb(frostbite) or respiratory issues.
How long have you had her and have you ever wormed her? Does she eat/drink at all on her own? Have you checked for crop issues?