It's like you taking a laxative. It's going to be a relief for her. I've done this flush and the patient was actually energized following each treatment. Chickens, in my experience, tolerate it very well.
Yes, it should fill the crop. Most crops can handle the half a cup. Bantams will need it adjusted downward as would young pullets. Tubing a chicken is like learning any new task - once you've mastered it, it's then a tool that you will find makes life much more convenient.
I've tried a number of treatments for vent gleet over the years, and have found that it's an extremely difficult thing to get rid of. Miconazole is mostly ineffective. The standard has been nystatin, which requires a prescription, but even that fails most of the time, at least in my experience...