Thanks for the responses. I'll reply to a couple of them, as some of this is stuff we've considered previously:
Tarps - we were really trying to steer away from tarps, for the noise/wind reasons mentioned, and because it's completely opaque. Also, it means more tensioning of the whole thing from the edges, rather than tying the covering directly to the arches & spines. Though I do wonder whether this (tensioning) makes the whole thing stronger, by spreading out external pressure across the whole surface... and whether that's a bigger win overall than having something the wind can blow through a bit. (w.r.t. rain - we'll have a tarp & bungees ready, as mentioned, but weren't planning on it being a structural element.)
Along the same lines, I wonder whether more arches & thinner pipe would be better at spreading the external pressure. I assume so, but I also don't like the way the super-thin huts squash & stretch. I believe we used 2" when we built a larger hut back in '98, but I don't remember for sure - nor do I remember the arch spacing.
Other odds & ends:
>> I would add a top spline.
There is one. May not be clear from the pic. There are three total.
>> -Suggest 20' tubing lengths, rather than 10
Can't do for transportation reasons. And our source for 10' is actually cheaper than 20'.
>> The over the arch tie downs should follow the arches
That's actually a bug in the rendering - they don't go all the way over, and there should be one at each arch, attached to the lower arch/spine junction.
I guess the arch spacing vs. pipe thickness is my biggest remaining area of uncertainty. Laying it out on the ground, it does seem like a 9.5' radius does make for a significant bend - maybe too much for 2" pipe? I still feel like those 1" pipes are like noodles,though - I was hoping on relying less on "correct" construction, and more on overly strong members, since we have so little time to make adjustments.