Hmmmm... this is a complex project here.
First off, for reference, here is the, imo, reigning mortarless oven build:http://www.pizzamaking.com/forum/index.php/topic,18944.0.html
This is one of the few (only?) builds to build more than just a box out of bricks. You don't have to mimic him, but I would read through the thread for inspiration.
You really want to stick to the 60% door rule. 12" on an oven this size, translate to 8" on the door, which might not give you good access to the whole oven. I might go 13, maybe even 14, but no higher than 14.
The name escapes me, but you want the special low conductivity firebricks that Jeff introduced to the forum for the hearth, if you can get them. Domestically, I don't think anything can beat those.
Can you find out what existing permits there are for the pit? That might help you understand what can exist there and if you can squeeze into that. The thought of paying inspectors to build a temporary oven causes me great angst, but, the thought of telling no one bothers me more. There's also a really good chance they might force you to extend the chimney above the structure- and attach it. Still, I think you have to talk to the authorities.
A Neapolitan vent is questionable for a mortared oven, but, for a mortarless oven, the logistics make it even less desirable.
I kind of like having the chimney a level below the main oven ceiling and the door a level below that, but, that was while I was still using the theories for the FGM throat reducer. Since the throat reducer is in flux, the theories are in flux as well.
Is the center of the limestone table supported?
Perhaps I'm not hearing you correctly, but I wouldn't put bricks straight on limestone- not that they'll crack- the bricks should ensure very even heating, but because you want a layer of perlcrete.
The oven has to be weatherproofed. The bricks cannot get wet, either during the cooldown or between bakes. If you're only going to bake when you're sure you're going to get two days of good weather, then you could bake, let the oven cool, then cover it with a tarp, but that sounds like a hassle.