That's similar to how my motion lights were. The chickens would set off the light at dusk, and continue to just hang out under the light until it got dark... then they were stuck on the ground because the coop was dark. So I think that's when they just flew to fence to get up off the ground, but sometimes I would hear them making a ruckus first, so I would guide them into the coop with a flashlight. It just made things run smoother after I moved the lights out of the chicken yard.
Oh, do you have a regular thermometer, or is it a hygrometer? It's more helpful to know the humidity level than the temperature. Chicken Wire... I know right?! It's good for holding IN chickens, but not necessarily keeping OUT predators. It'll probably forever be a source of confusion.
Yes, ventilation is just as important (if not more) in the winter as any other time. Trapping heat inside actually creates moisture. Just body heat and breathing makes a very moist environment which can bring on respiratory problems and even frostbite. No coop should ever be air tight. Using Sweet PDZ can alleviate ammonia and smelly issues. Lots of vents placed above the roosts will keep chickens out of drafts and let all that moisture escape. A dry coop makes for happy healthy chickens!