Thank you, your welcome....And I'll do my best.
With such a slight radius, if the Spandex is stretched equally across the Frame, with just a slight adjustment "directional-wise", it will conform to the curvature even better than a material without such 4-way stretch potential. In any case, it's easily adjustable. It won't take much effort, or a really significant difference in tension.
Since I do not know what materials or method you are using / have used to construct the frame, I'm limited in what suggestions I can provide. I can relate that if a Flexible Frame is used, and the material is stretched across it first, when it is bent into the Curve desired, the Spandex easily conforms equally to said curvature.
A great PJ, that. More on that below....
No.....White over White. You need none of the additional Contrast enhancement Black would provide, and two layers of White will effectively mask any surfaces behind the first initial layer.
Also, White on White will maintain the maximum allowable Gain that Spandex can provide.
And you should. By availing yourself of the Lens Shift, you should mount the PJ as far forward as you possibly can tolerate. Again, this is to preserve your available Lumen output, and therein the gain off screen.
Looking at all the above suggestions, if you place the PJ forward to 14'-3" to achieve 20 fl (18' would garner only 16 fl ) your results watching 3D in a totally darkened room would be very acceptable.
Following just behind it's fantastic Audio properties, the one other thing that helps justify Spandex above all else it it's smoothness. Using DLA optics, the last thing anyone would want is seeing any vestige of surface-induced texture. XD material just isn't in the same league in that respect. Virtually nothing "AT" is for that matter.
Hey...I got'cher back, Jack.
(...I couldn't resist....