18" diameter x 5" high stove pipe doesn't exist. If it did, this would be WAY easier. That's for the most part, what you're creating- what you're piecing your small strips of cut stove pipe into. The only difference is that your finished 'pipe' won't be a full circle, but, rather, a U shape with an opening in the front where you launch the pizza.
5" high stove pipe doesn't exist either. You might find some sort of 5" adapter or extender, but I wouldn't waste your time looking for it. Here's what you want:http://www.homedepot.com/p/t/202907227?productId=202907227#.UbFdEmoub0k
This is 6", you may want 8". It depends on the wall height you go with. Earlier you spoke about tucking the wall between the brick and angle iron, but now I see that you have the wall sitting on the brick in your diagram.
If you sit the wall on the brick, and go with 5" walls, with .5" quarry tiles, that's 4.5" to the ceiling. It's tight, but think that's what I'd go with. You don't want to cramp the launch, but, at the same time, in bottom heat scenarios, the ceiling, for proper heat balance, should be as close to the hearth as possible.
If you tuck the wall between the firebrick and angle iron, that means you have to add the 1.25" thickness of the bricks to get the same 4.5" dome height making for a wall height of 6.25". There's one consideration with a tuck, though. If you're at angle iron level on the sides when it comes time to run the wall across the back the two angle iron pieces in the middle will block it. What I'm think you can do, though, is terminate each wall run at the back middle.
I've been thinking a lot about my bolt post/film projector idea. First off, I'm renaming it my wicker approach. The angle iron lip that the firebrick sits on has enough room for thin bolts to run up through it while still allowing enough space for the brick to sit on. Beyond putting bolts at the termination points, you can put bolts every few inches (maybe 4") along the path of the wall and just zig and zag the wall through them like a wicker basket. At the end, you can loop the wall around the post and fold it back and slide it into the previous post.
One of the strengths of a wicker approach is that, should the seam of the stove pipe be either riveted or spot welded (a distinct possibility), rather than clasped, and can't easily be reclasped, you can leave the wall sections separate as you thread them through the posts, overlapping them at the ends. You might even position your bolts/posts so that those are where the ceiling angle iron crosses/sits on, allowing the posts to take the brunt of the ceiling weight rather than the sheet metal from the pipe. The bolts will be the weight bearers, while the pipe strips are just air entrapment.
The only potential tricky part to this is the flexibility of the stove pipe. Flashing would probably wend it's way through the posts and double back beautifully, but stove pipe might not be quite as flexible- at least maybe not for the double back part.
Other than the decision as to how to approach the wall, your diagram looks right. The only things I would change would be, if you are going to sit the u shaped wall on firebrick, without posts either in the middle or at the end, then you'll need to add right angles at the top of the U/front of the oven and corresponding bricks for the lateral pieces to sit on (see my original drawing). You could, in theory, fold the walls inward, but then you'd lose precious lateral real estate on the pizza size you can make. Right now, you can do an 18" pie. You might not do that all the time, but you want to try it at least once.
Speaking of real estate. The diagram has your quarry tiles arrange in a rectangle that looks about 12 x 22. They don't need to extend all the way to the front of the bricks, but you definitely want them as close as possible to each wall. The center pieces of 1/8" angle iron, when back to back, will give you 1/4" in the middle, so your quarry tiles should be 18.25" x 18.25"- a nice big target for 16" pies and just enough to squeeze an 18 in there.
I think you're clear on how to cut the stove pipe with tin snips, but, just in case you're not, here's a picture. Btw, tin snips can make very sharp edges. Use leather gloves and sand all your edges down. Oh, and I think this is obvious, but don't use the crimped area.