For the first try I think I would just go straight down. It not going to be an ideal situation for draw as is, so I would try and avoid bends to start. I also didn't mention, make sure the bottom of the extension is above the top of the door. If it's lower most of the smoke is gonna roll out the front. What size is the flue? Pictures can be misleading but if it is single wall it looks pretty decent size compared to the rest of the oven.
As for the heat under the oven, my ovens are generally less then 5F above ambient outdoor temperature on the bottom. Even if you don't get to that point 220F is way to high. It almost makes me think there is some sort of thermal bridging going on due to poor design on top of a lack of insulation. You would probably kill two birds with one stone(I assume the dome is pretty tall) by pulling up the floor and adding some cal sil board then re-installing the floor, but that seems like a lot of work for an oven you just bought.
I think I would personally ask the manufacturer where you can see someone using on of their ovens to produce neapolitan pizza. Can they demo it for you? Put the burden of proof on them, because their website seems to show only one of these mobile ovens and the only pictures of pizza are highly unacceptable. Makes me wonder if you have the only one they have produced yet they are insist their unproven oven works.
Thanks for the flue suggestion. I think I could muster up just such an extension even if only temperary so that I can then test the heat performance again. Would you suggest it comes down to 63% of the dome height in a vertical line of angled towards the oven entrance if possible. The other issue is the temperature of the oven underside when baking. Mine was 220 F, if this is too high can I ask in your opinion what a normal/acceptable temperature would be?. Any thoughts on the suggestion of lifting the bricks and adding a thin but effective additional layer of insulation and if there is a material that would be suitable?.
Thanks again for your help.