Ventilation is tricky in such a small space. I agree that it's important. If the back opening is the same size as the front, what I would do is block off the top half of either side so when the birds are roosting they are out of any draft or wind, leaving a couple feet of ventilation for air exchange on either side. In an ideal world you would have the ventilation above the chickens and under the eaves, but it's better to have them out of any drafts than to have the vents higher up IMO.
Unless you're hatching eggs, the chickens should be able to jump up 20" into the nest boxes and down safely, especially if you have a few inches of good soft bedding in the bottom. You might want to put a bar in front of them a few inches lower that makes it easier for them to jump up onto but mine jump 3' nearly straight up onto my roost bars in my coop. I would consider two nest boxes just because your birds breeds are on the large end, but also my nest boxes were about 1.5' squares and were PLENTY big so you may be able to do two nest boxes on one side and just screw some plexiglass in on the other side, giving them some light through the window.
I live in northeast Ohio, so only a little bit warmer than you, and I've never heated my coops, nor had any serious problems. (Some light frostbite on the biggest chicken comb that went away without incident.) If you do want to heat your coop, use a sweeter heater or something similar, not a lamp. I also don't do artificial lighting as I think the winter rest is important for them, and I treat eggs as a seasonal food as a result, but that's a personal choice.