Yep. I would almost bet money that the metal fence is your problem. So sorry. Can you wrap the metal in something cozy like a towel or blanket to prevent more of this?
I feel pretty confident we’re looking at frostbite. What is on the floor if the barn and what do they use for roosts? Have read a couoke posts lately of frostbite from a concrete floor in a garage. I think you are handling it well. I agree you should probably address the scaly leg mites, as well.