The egg with provide sufficient Selenium.
Just look for "B-Complex" it should list 8 different B Vitamins on the back label. It's o.k. if it also contains vitamin C.
The neck upside down is a neurological symptom.
Poultry Cell is good, but when you can, get some Vitamin E (400IU) and B-Complex. Give her 1 Vitamin E cap daily along with 1/4 tablet B-Complex. Give her a little egg to help with the uptake of E.
Is her crop emptying overnight? I'd re-check...
There's a lot of fibrous material in the poop. She only eats chick starter? Is it a loose grain feed or commercial crumbles?
If possible, get a fecal float to see if Coccidiosis and/or worms are part of the problem.