Chaz, mill scale is just iron oxide- which is no different than the iron you might consume eating with cast iron cookware. That being said, most people take it off. The easiest way is an overnight soak in vinegar and then a good scouring.
If you don't take off the mill scale, I'd still definitely give it a good scrubbing with soap and water. I've been pretty anti-seasoning in the past, because I haven't felt like it would be stable at pizza baking temps, but I'm seeing lower temp baking plates (such as Marc's aluminum) take on and maintain seasoning, so a very light layer of vegetable oil wouldn't hurt.
Is the Conagra flour bromated? If so, I don't see any reason for the AT whatsoever- other than using it up.
That formula looks good.
Definitely- go with a 500 pre-heat. Position the plate on a shelf about 5-7 inches away from the broiler and turn on the broiler for part of the bake. In my 4.5 to 5 minute bake, I turn on the broiler at the 1:30 point and leave it on until the end, unless I feel like the top is taking on too much color and I shut it off. It takes some trial and error to get the broiling right.