The birds will peck at the paint and any "spots" that appear on it, especially as composite/chip board starts to loosen up over time, lots of cracks for dust to settle onto. I like tile board (basically 4 x 8 sheets of whiteboard) which is pretty cheap, very smooth, and anything that the birds do NOT need traction on, I make out of that. I would suggest sheathing the interior with that. I've also had the birds consume any insulation that has been waiting for the next project. Rigid foam-board and blanket type, but I haven't had any fiberglass lying around at least! "WHY?" I've asked myself, what in nature is that yummy and has the texture of that? Of course our birds are pretty far removed from jungle fowl...
You said you've got lots of ventilation, but only talked about sealing the windows, etc. up tight, I assume that is because of where the windows are right at the roosts. There is a fantastic article on BYC about ventilation and the general guideline is 1sqft per chicken. I'm in northern VT, so cold and lots of snow as well. I've never had frostbite issues, but I did follow those guidelines.
You could also send roost bars to the back wall (like an E with 4 "fingers", adding probably almost 8 feet of roosting space where you won't be needing to stand. Obviously you know that 2x4's flat are the best roosts!
If you just changed the bottom attachment of the ladder, it would be fine. Lift the lower end up 18" or so, they can hop onto the ladder then walk up to the roost. Looks like you've got enough space between the treads on the ladder to get an ice chipper or wide putty knife in there to scrape which is pretty easy, so even if it catches poop, won't be much of a big deal.
From the tool brands I see in the photo, you either do your homework, or know building! Nice framing and general construction techniques too!