Well, 12" from the wall is pretty close. OK for bantams but reeallll tight for regular sized chickens. Also, a 2x12 will miss a fair bit of poo, even if it is centered under the roost. (And if it is centered under the roost, and this is all above your nestboxes, some poo may tumble/squirt into the opening of the external nestboxes below.
If you have banties and/or are *forced* to have the roost that close to the wall, fine; otherwise you might consider moving it out a little. And, maybe see if you can find something wider for the droppings board, if not now then at least "when the appropriate material becomes available". IME 16" will catch most but still not quite all poo (this is from regular sized chickens).
As long as no significant am'ts of poo are squirting/tumbling into the nestboxes (as per my concern above), there is nothing wrong with having the roost and droppings board over top of the nestboxes. I actually am set up that way, with regular inside-the-coop nestboxes, and it works fine for me. You just need a wide enough droppings board to keep the boxes clean.
Good luck, have fun,
Pat