Michael, I think the 9" dome is a good choice. That's what I would go with. There's been copious discussions about hearth to dome ratios, but, at the end of the day, I think, for Neapolitan pizza, it's better to err a bit on the low dome side than on the high.
I'm not sure about the downward sloping roof. Are you using this as a way to remove the throat? The way you had the throat before- that was excellent, imo. The throat just needed insulation. If it's a sloped roof + similar throat, I guess that might be okay- when you say 'sloped' you really mean 'stepped' roof, correct?
If you're taking the chimney route, which, imo, along with the increase in size, are the primary goals of this revamp, you really need to do the chimney right. Chimneys are all about laminar flow. No lips or protrusions. The 4 x 4 opening leading to the 6" pipe? That's not so good. If you've got a 6" pipe, you pretty much need a 6" opening so the gasses vent smoothly.
I did some research on this last night and think I came up with a possible solution. Basically, you can buy rectangular clay chimney liner that matches the dimension of firebrick (4.5" x 8.5"):http://superiorclay.com/flue-liners.php
This way, your chimney replaces one brick. The 1.25" lip on the angle iron gives you an opening of 6.5". If the liner is .5" thick, that means that the opening for the liner is 7.5". The 1/2" overlap (on both sides) of iron isn't ideal, but, I think, once, you get the position of chimney you want, you can take a hacksaw to the angle iron, and, for the section where the chimney sits, you can cut the lip to be .75" rather than 1.25". That will ensure proper laminar flow.
One other nice thing about the 4.5 x 8.5 liner is that it vents a tiny bit less than 6" pipe, but not dramatically less (23 sq. inch vs. 28- the equivalent of 5.5" pipe). It should work well for this size oven.
The extra thick angle iron is pretty sexy, but... if the angle iron is rusting out because of the heat, the extra thickness is only buying you time. From the photos of the old iron, though, it looks pretty good. If you really want some extra piece of mind, try lathering the old angle with some naval jelly. That will eat away at the rust and show how much of the metal you lost. This has been exposed to the elements as well, correct? Are you planning on weatherproofing mortarless 2.0?
If you ever do major entertaining, sitting at the oven can be a bit of a drag. It sounds like a little late for this, but if you want to watch a fire burn, you can get fire pits for very little. Instead of looking in a slit, you can watch the glory of the whole flame in a pit. With a fire pit serving fireplace duties, you can raise the height of the oven and use it at shoulder level. Just an idea
If this oven can do what I expect it to, under $800 will be a steal. I think, thanks to the pioneering work that you've done, that the people that follow in your footsteps might be able to trim off a bit of that expense, considering that they won't be going through the different iterations.