I am so, so sorry for your loss... Poor girl...
Honestly, sour crop is almost always a symptom of a more serious illness, like cancer or a reproductive disorder. In my experience, it's usually so advanced by the time the crop is affected that there isn't much you can do. And our beloved chickens don't show signs of illness early enough for us to recognize them because they are prey animals. I've lost two similarly, and it's always devastating. I sent one to be necropsied to make sure it wasn't something communicable. That is always an option if you want to know what happened.
Again, I am so very sorry you lost her. You did your best to help her, but there probably wasn't anything you could do.