Hello, and welcome to BYC!
You can give Nutri-Drench, just for no longer than a week or so every day, because like mentioned it can cause diarrhea. I would say twice a week would be fine, in my opinion.
I concur! If they aren't sick, then twice a week should be okay. We actually use Poultry Cell about once a week here. These are silkies I worry about vitamin deficiencies with.
Welcome. I think the key to your question lies in the question. You are referring to a supplement for winter weather. If you have chickens that are sick or struggling, then give them heat.