You did the right thing in covering the vent right by the roost. Drafts are not equal to ventilation.
Please do not use this information to win a fight with hubby.
You should have one square foot of ventilation per 10 square feet. I would keep the vents open. If there is frost gathering anywhere, add more. Keep water and warm foods out of the coop.
How bad is the frostbite? Minor frostbite is bound to happen once temps get to -4°F or lower. It's normal, and as long as it's only affecting the tips, it's totally okay. If your girls are comfortable and acting normal I would
not make any modifications. I would wait, and place a hydrometer in the coop. I don't know what the optimal percentage of humidity is, but I'm sure someone can help-
@Blooie @junebuggena @aart , what is the optimal humidity percentage in a coop that is in a cold climate?