After reading through this entire post, I am in tears! The OP had tried so hard to keep this hen alive. Her dedication and detailed documentation of every efforts were simply amazing. More so was that @casportpony (Kathy) stayed on offering guidance every step of the way.
The reason why I was searching under this topic was that today, I found my favorite hen "Yellow" looking very lethargic. She sat inside the nest box next to the golf ball (not on top of it) no matter how many times I moved her out of the coop or put her on the roosting bar at night. She just keep returning to the nest box no matter what.
She is missing a large patch of feather on her abdominal, not sure that was due to molting or feather eating. Our small flock of four 1-yr old White Leghorns hasn't have any feather plucking issue before. "Yellow" was not responding to the shake of the treat jar today, which was unusual. She usually runs to me, flies up to my arm and eat out of my hand.
I checked for mites and lice, didn't find any. She just laid an egg at 4 PM today (very late in the day), doesn't seem to be egg-bound. Her crop is soft, poop seemed normal and solid. No symptom of respiratory issue. All four hens were dewormed every 6 months with Valbazen. The last round was in May 2015, so it shouldn't be worm. Maybe she ate some wild mushroom in the yard? I am at a lost on why "Yellow" is so lethargic. She ate a few pieces of cooked chicken tonight as a protein boost, not much else. Tonight, I moved her to inside the house for observation since she was sleeping inside the nest box.
I thought white Leghorns usually don't get broody, so there is something wrong, but I have no idea what. I hope she is not getting sick because I am really not very good with nursing animals, but I hate to lose her, as she is my favorite. Help and suggestions are appreciated.
Hens go broody when you don’t want them to… and won’t go broody when you do. 
