We are on the same page
Top hinged windows are best opened to the outside, to deter with rain infiltration, especially where there are no roof overhangs, and to not add interior obstructions or landing/perching places. But that window is so small and your coop is roomy it might not matter and it will be easier to install with window opening to the inside.
Poop board/roost will need to be attached to stud, but size does not have to match studs. I built simple support attached to studs. Poop board bottom is luan and is lined with heavy duty vinyl held down with corner trim. Poop board itself is just tacked thru back edge into stud in 2 places.
I built my roosts to be removable but found I didn't need to remove them to clean.
Can reach under the 8" clearance with hoe and sifter basket.
http://www.backyardchickens.com/t/6...raphic-gross-poop-pictures/1100#post_13179595
The never ending debate about what size to use for roosts goes on and on,
my advice is use what you want. My birds roost on the 3.5" of a 2x4, and on the 2x2 edge of poop board,
and on the poop board itself. Never had toes freeze or damaged tendons or broken keel bones.
They probably
will try to land on those ledges....good thinking.
Chicken wire would be better...or, oh!, even maybe just a couple strands of wire at 3-4 and 6-8" above the ledge would deter their landing there and would not collect debris.