I think your coop looks just fine as it is. I would concentrate on getting a nice chair, and a good hot cocoa recipe for when you sit and watch them. All those 'cracks and crevices provide ventilation. If you are worried about predator intrusion cover them with hardware cloth, but I would not seal them up. The only place I used caulking is on the floor so if need be I could wash the floor and not have it get the ground underneath wet. You really can not have to much ventilation, I personally do not worry about the whole 'draft' thing either, kinda silly in my opinion. But to each their own, not right or wrong, just different.
I personally don't believe you need heat, When we had horses that were in trouble during the winter, we used I.R. (red) heat bulbs that had hardware wire 'tubes' around them to keep them from being knocked around and broken. Never had a knocked out bulb. You can see what I mean about the 'tubes' on my page. Easy to change the bulb and if the birds hit them the bulb will not break. In my opinion, those new ecobulb things should never be allowed in your coop. They have mercury in them, which if broken and released into your coop, goes straight into the eggs. Break one of those in the coop and you have contamination at a level that the EPA considers to be hazardous enough to require special 'protected' cleaning procedures. I hate those darn things...
FWIW- it is 12 degrees at our house, no heat in the coop and snow outside. When they get let out, they run out, eat snow and play. Soon it will be well below 0, and we still wont add heat. This is just how we raise our birds, not right or wrong. I just think it is important to point out that they do not need supplemental heat even in temps as low as -30, and I believe they are better off if you don't.
