B12 is not what you need. It has vitamin b 12 and vitamin k, but nothing else. You need something with b2 (riboflavin.) Get the Poultry Cell, identical to NutriDrench, but it has riboflavin. Or get Human B Complex, which you can crush or dissolve in food, 1/4 tablet per day, and you can use...
Egg binding can be a cause of temporary lameness. Is she pooping okay? Just feel inside her vent to check for egg binding. The curled under toes can be a sign of riboflavin (vitamin B2) deficiency, but also can be a sign of Mareks disease. I would give her some human B complex crushed into food...
What does she do if you lift her up to stand? First though, check her legs for any bruising, sores, swollen joints, or displaced bones. Look at the bottoms of her footpads for any scabs or signs of bumblefoot. Look for curled under toes. Chickens with Mareks can move their legs, but the movement...