As for your venting, if you prop open your roof, you will need to use some sort of screening. There are all kinds of things that can get in even the tiniest holes...mice, rats, snakes, weasels, etc....things are going to come for miles when they smell chickens. Some of them are looking for eggs, and others the birds themselves. So always, always close every gap at night to keep these nighttime things out. They have a lot of time on their hands and will work at something til they get in.
Yes, 2x4 with the 4 side up so they can keep their feet and toes underneath all those feathers and their breasts. This will keep the heat in the birds.
Chickens do not grip the roost bar like other avians. So they will enjoy sitting flat footed on the bar. Chickens are very suspectable to bumblefoot and any puncture wound to the pad, scratches, sharp stones or thorns, even jumping off high places and bruising the feet will cause bumblefoot. So keep everything smooth. No sharp ends of wood screws or nails, nothing sharp to step on and for the heavy breeds, no jumping down off high perches. Occasionally a bird is going to turn up with bumblefoot. I have dealt with it a lot here because of the prickly ground. Catch it early enough and sometimes you can just keep it bandaged for a few weeks and it heals on it's own. Sometimes a surgery is needed, but it is pretty easy on chickens since the pad is so large.