Any opinions as well about whether using that design without a hen house floor would keep the chickens warm enough?
I can't answer any of the engineering things, but we don't need to keep chickens warm. We just need to keep them dry and out of the wind and their feathers keep them warm.
I'm not understanding from your description if the walls around the roosting area go all the way to the ground or not. If they do, then there's no need for a floor because there won't be any way for the wind to come in underneath them.