The sugar water is important. That will revive her if she's just weak. That will give her a lift and hopefully, she will then eat on her own. You won't need to stuff food into her.
Before tubing, it's important to try to get the patient to eat on her own. Understand that by tubing food, it goes directly into the crop and immediately makes the appetite, whatever there is, disappear. We want to encourage her appetite so she eats normally. This would get her back in the habit of being hungry and having the urge then to eat to satisfy it.
You're at a critical juncture with your patient. Do the sugar water, watch for her to become more energetic, then tempt her with her favorite foods as soon as she shows signs of the glucose reviving her. Continue this all day.
In the morning, you will either see an improved patient or a patient that has not responded and is declining instead. Then you will assess the situation and follow your intuition as to her chances of any improvement.
When facing having to euthanize a chicken, the hardest part is making that assessment whether you're seeing decline or improvement. Making the decision to euthanize is hard, but after that, you shouldn't try to second guess your decision. Carry it out quickly without thinking about it. Afterward, knowing that her suffering has finally ended will be such a relief, it will balance the grief you'll experience. That grief is acute the first day, less the second day, and after that, it gets easier to bear each day until all that is left is your good memories of her.