Urates are the uric acid crystals excreted by the kidneys, and the liquid is excess water that she is drinking. Some of the fluids are coming from the bowel. Do you have a dark closet, basement, extra bathroom, or garage where there are no windows? Her feed does not have to be reduced a great amount, but the protein can be decreased, along with her daylight hours. A vet can insert 2 temporary stitches in the vent opening to hold the prolapse in, but the dark scabby material first needs to come off.

to you and your chicken both! I'm so glad you caught this early; you're doing a great job and I have high hopes for your girl. We recently lost one to this, but she was much worse off than yours when we discovered the problem and we couldn't save her. Following to learn, in case there's a "next time."
We have had what feels like a small setback this morning as it looks as though some of the pink flesh exposed last night has now got a thin grey layer on it again. I tried peeling some off on my own and was successful until I hit a bit that looked very close to bleeding. Tonight we will try again during her bath but make sure to apply copious amounts of Vaseline/Petroleum Jelly afterwards to make sure that the area is protected against her dribbling urates which I suspect may be burning the tissue and causing the problem. Otherwise, maybe the exposed tissue isn't quite ready to be exposed and thus undergoes some slight scabbing again.