You can put bag balm on their combs and wattles to help avoid frostbite.
The ventilation is a huge thing indeed...without proper ventilation, frostbite is only ONE of the many issues to come. Respiratory illness, disease and death are some of the others.
Cut some openings near the roofline, cover them with hardwire cloth and the more the merrier. In the winter, cover th northside ventilation if it is staying too cold inside the coop.
Fixing frostbite once it has occurred is difficult. If you notice the frostbitten areas turning dark, they will eventually die and fall off. Parts of the combs, the combs, toes, etc etc.
Good luck with them.