Straight flat sawn is better, I think, but whatever you can get with the best grain is best. Quartersawn bellies are slightly weaker than flat sawn in compression because you have more winter rings making up the surface, but the difference usually isn't huge. I ignore it on tropical woods, for instance. But, it's possible to cut QS boards that look perfect, but violate the radial grain.
Your curves may not be as bad as they look, at first blush, for one thing. In your top pic on the right, a little planing on a couple of those long flat-sawn slabs should get you a ring you can follow or a perfect flat sawn back. And I don't see why you couldn't back with bamboo. For that you still want good flat grain,but it doesn't have to be as "perfect" as a board stave.
I once found a flat sawn RED oak 1x6 board where the entire thickness at the middle was TWO growth rings, really one and a half. The face had barely exposed the biggest ring, so I chased the big ring, literally 1/2" thick, which was barely crowned at all. It was WAY heavier than any oak I've ever held and made an absolutely awesome 55 lb pyramid bow, 2.5" wide, that took like 3/8" of set.