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Neighbor
1) Don't amend the order, you would regret it later.
2) Go with 4'
3) Chickens can't read and they don't have rulers either.
4) 6" probably is more than enough. And that is 6" above the floor of the nest. Bear in mind that young chickens may not roost for quite some time. Especially those lower in the pecking order. You may find that some will roost and some will hang out in the nest boxes. They will also poop in there. I recommend that you do not put nesting material in the nest boxes until they all are on the roost. It may not discourage them from sleeping in there, but it will make it easier to clean.
5.) My top roost (and the favorite) allows the girls to stand and their combs brush the ceiling. It doesn't seem to bother them.
6) Sometimes, the chicken butt doesn't extend over the roost at all...as evidenced by the poop on the roost.
My roost bars are 10" apart and there are no issues with over crowding.
In regard to venting, moisture is the biggest issue in a coop. With moisture comes odor. Here in Iowa you have to find the balance between venting and drafty. I like to keep in mind that they are birds and come equipped with a pretty decent down coat. This past week my chicken door has been open to the sub freezing temps. Not only did the girls not complain, they went out side. If it gets to drafty, they will let you know. Keep an eye on them and they'll do well. Since they will be arriving in March, venting will not be needed until they get a bit older. You may need to control the moisture by changing the bedding, but they will need heat until after they are fully feathered. It is my opinion that you will figure out if you have adequate venting before next winter. As for summer, make sure they have a place that provides shade where they have access to water and they will do fine. My coop is not like yours and mine never gets above 85*. The girls are rarely in there when it is that warm...they are under the coop or in the flowers where it is shady.
You can rarely over think the construction of a coop in my opinion. This is a great place to get info, and I find that there are MANY MANY opinions from different perspectives.