Again, B12 is not B complex, just b12 or colbalamin. B complex has all b vitamins, and the important ones for chicken ps is riboflavin and thiamine. Read the label to see if that is what you have. Great advice about the slings.
So sorry for your loss. I would try to make the 2 survivors comfortable. Offer water with electrolytes up to beaks to drink on own or drip some into beaks. Watery chicken feed made fresh could be given to increase fluids. Vitamin E 400 IU can help with neck injuries. I’m sure once the hail storm...