Welcome to the forum!
First of all, I suggest you read this page about ventilation. It's an excellent reference:
https://www.backyardchickens.com/web/viewblog.php?id=1642-VENTILATION
Bottom line, I think you need a lot more ventilation than you're thinking about right now. I would not recommend that you try to use the greenhouse vent fan; it will quickly become clogged with dust from the chickens' feathers.
Be sure to cover up the foam insulation anywhere chickens can reach it, otherwise they'll eat it.
10 bantams in a 24 square foot coop is only 2.4 square feet per chicken. That's kinda cramped. I think 4 square feet per chicken is more of a minimum as a rule of thumb, and I use this number for bantams as well as standard size hens. You might be able to get away with it without too many problems if your weather is good year round and your chickens won't be hanging out inside the coop all day because of bad weather in the winter. But if your winters are severe, that stocking density could be a problem. If nothing else, keeping so many birds in that space will mean you have a lot more work to do as a chicken keeper to keep things clean enough to be healthy and not stinky. Maybe they'd let you change your order down to 4 or 6 bantams?
I am not a big fan of ladder style roosts because they create more squabbling at night over who gets to roost on the highest rung. Everyone wants those preferred spots. I have all my roosts on a single level, and I build about 12 inches of width per chicken. I also have bantams, most of them tiny Serama crossbreeds, and a couple of d'Uccles.
One issue with repurposing a greenhouse is "the greenhouse effect." In the summer, you do need to be careful about the inside of your coop getting too hot. Chickens can handle cold far better than they can handle heat. Anytime the temperature inside a structure gets up into the 90's (which a clear roofed structure in the summer sun can do pretty easily), it gets dangerous for chickens. You'll probably want to have even more ventilation for summer use to allow heat to escape and cooler breezes to blow inside the coop, and you'll probably also want to rig some shade over the roof to block the sun from entering.
p.s. Forgot a couple of your questions. Usually, it's recommended to have 1 nesting box per 3 or 4 hens. They tend to like to use the same one anyway. It seems to me a bother to build external nestboxes because I always go in the coop anyway to let the flock out in the morning and to lock them in at night, plus I like to visit with the girls. But it's basically a personal choice. Both ways work.
