Have you determined that River does indeed gave sour crop, Lozzy? For some reason, I thought I only did the cream twice a day. What is the active ingredient and percentage? I might be able to find dosing info. Do you know how much she weighs?
Yes ACV is 1 tbsp per gallon. I personally would not stop layer feed yet. Is she laying? If she is definitely not laying, you could substitute some other foods (kale, meat, egg, a tiny bit of unsweetened yogurt) for a couple days, but a girl’s gotta eat to maintain her energy. I find their crop sometimes slows down during molt when they like to starve themselves.
Shad has good advice on handling often. It always seems the most difficult to handle get sick! Also good advice on seeking avian vet help if that’s an option. Do you not want to post in the emergency forum?
Also, how is her behavior, appetite, etc? And have you checked for crop function? I don’t know that I’d start treatment unless I knew what I was treating for.