re: vertical seams....
I'm not a designer by any stretch but I know fabrics pretty well. The bigger the expanse of unseamed material the tighter you can stretch the tarp. A well made flat-felled or folded french seam would withstand the stresses just fine. But you will not get as tight a pitch with vertical seams that break up the area. If tightness is not an issue to you then there is no reason not to do it. But in the snow you can get accumulations that may weigh the tarp down in places and cause sagging that you would not experience with a more tightly stretched tarp.
Give it a try... but I would not sink major bucks into the sil or cuben until I'd seen how a prototype behaves in the field.