From your description, I'm guessing somewhere around North Carolina or Virginia. If so, you are correct that the chickens will be roaming the larger area most of the time, even in winter. You get what, maybe 4 or 5 days of snow a year? I was raised in the hills of East Tennessee and the door to the hen house was never closed. There were occasional stretches of days that it was very cold, windy and wet and they did not roam much, but we did not have any real problems with cannibalism, I think because the hens could escape if necessary. They were not fenced in at all and they pretty much stayed within a couple hundred feet of the hen house all year round. And a few of the hens roosted in trees outside the hen house, even in winter.
I'm assuming you have full-sized birds, not bantys. For 25 hens, I'd use 3 each 8 foot 2x4's for the roost. If you have them side by side, separate them by 12" horizontally and at least 6 inches from the walls. They can crowd together if they are cold and separate a litttle if they are too warm in the summer. In colder climates where frostbite is a big risk, many people put the 2x4's flat which forces the birds to cover their feet with their feathers when they roost. In your climate, you'd be OK with the 2x4's on end, especially considering how hot some of those southern nights can be. If you round the corners off, it's a little more comfortable for the bird's feet. And in the really cold climates, the coops have to be insulated. If you are where I think you are, you do not need to insulate, but you do need good ventilation. You'll have that in the "coop" area for sure, but make sure you get it in the roosting area also. If it is not well enough ventilated, the ammonia in their poop will give them respiratory problems.
The birds need to stay dry and be able to get out of drafts. Whether or not your two enclosed-sided, covered 10' x 18' coop will work will be determined by your micro-climate. Where I currently live, in a valley in northwest Arkansas, I get either a north wind or a south wind, sometimes with a westerly component. A two-sided coop would not work for me, but a three-sided one might as long as it was open to the east.
The nesting boxes do not have to be in the roosting area. We had hens that laid in the hay barn and roosted in the hen house and they were about 80 feet apart. There are a lot of threads on nesting boxes on this site if you need help or you can start your own specific thread.
I think terminology may be causing some problems here. The traditional nesting/roosting/stay warm area is the coop. The wire-enclosed area outside the coop where they take the sunshine is the run. What I think you are proposing is a separate roosting area, which could be rather small, no problem. The nesting boxes could certainly be separate. You have a wire enclosure outside but the gate is open and they have access to a large fenced area inside the goat fence. What I'm having problems with is the stay warm area.
If chickens have access to a large area outside, then the experts suggest you give them 4 square feet each in the coop. Since you are going to totally enclose their roosting area anyway, why not make this 10 feet x 10 feet? The roosting area is going to be about 25 square feet anyway and you will want to give yourself enough room to clean out under the roost, which will make it evan larger. If you feed and water outside this enclosed area, the birds will not spend much time in this area other than when they roost. That means more poop is distributed outdoors so you don't have to manage it as much and you're not getting the floor wet.
