The y-600 has adjustable dampers? I stand corrected. After reading this, I just did some digging on the Marsal. Apparently that has dampers as well, so I've been wrong about that also. Regardless of my incorrect information, I still strongly believe that, with the bake times you're working with, you're never going to want to dampen/limit any heat reaching the top of the oven, so, in your case, I still don't think dampers are necessary.
Your firebricks are a little 'brittle and crumbly?' I'm sorry to hear that. I think that rules them out. If a piece breaks of and ends up on the pizza, someone could easily chip a tooth. Are you sure they aren't just a bit sandy? Sandiness can generally be washed off.
Don't be too concerned with larger firebrick dimensions, by the way. As firebricks get bigger they can be less sturdy and less flat and the lateral heat transfer on large whole bricks is really not that much better than smaller bricks, as long as the smaller bricks are butt up against each other.
This is a bit outside of my current skill set, but perhaps, with your troubles tracking down firebrick, you might be able to cast your own fibrament-ish hearth. I know they reinforce it with something- most likely fiberglass. If I were doing it, I might opt for stainless pins. With the right cement mix and perhaps something along the lines of a homemade vibration table...