I wish I was there to feel her crop. Yes, I'm an advocate of crop surgery when all else fails to clear it. However, surgery is for impacted crop, not sour crop. When we resort to surgery for an impacted crop that hasn't responded to oil, stool softener and massage, I can instruct the chicken care giver in performing surgery, and the results usually produce a clog such as you'd find in a shower drain. The surgery is probably not that painful for the chicken since only a small one inch incision is made on the chest wall, then another similar incision is made in the crop wall. Hook a finger inside the crop and pull out the clog. Super glue the incisions and that's about it. Well, there are more details, but not now.
You are the only one that can determine if the crop on our patient is impacted or simply sour. If you check her crop in the morning, it will be easier for you to determine if there's a "shower drain" clog in there. If there is, and you wish to try the surgery, I will give you a video and detailed instructions.
If you wish, we can try the amoxicillin on our little girl. There's no way to know if she has an underlying condition that's treatable with an antibiotic, but at this point, I feel it does no harm to give it a try.
The only downside to using an antibiotic at the same time a chicken is fighting a yeast infection is that the antibiotic can sometimes aggravate the yeast condition. If you give a tablet of probiotic each day, perhaps that can mitigate the effect of the antibiotic on the crop.