Michelle, we don't have too many days when it hits 100 degrees, but my barn was not shaded by trees and it was a corrugated, galvanized sheet metal; wide open on one side and a very large open door on the opposite side, but could still get quite hot. If everything's in the shade, maybe no problem. Some of the plans I saw for rabbit barns included huge exhaust fans in one end of the building, and the wet fiber material in the other end; i.e., made a big evaporative cooler out of the building.
And yes, rabbits normally nurse once or twice a day. I'd like to see your "properly constructed" nest boxes. Of course, one way is to have them recessed in the floor of the cage. And of course the bottom of the nest box needs to be ventilated to allow the babies' urine to dry. And I think most folks use hay for nest material, but I found that newspaper run through the shredder worked even better (not a cross cut shredder but simply the 1/4" shredder).
You always want to have fresh water available. Feed is another matter. If you feed them too much, they get too fat and refuse to breed.
450-800 does? I don't see how two people can handle that, even full time. One of the books I read said 100 does and two people full time might make $30k a year.