Not really, though it is a mechanically and thermally fragile way of getting enough bandwidth for three SSDs. I’d use the two x2 slots for ASM1166 cards and put a fan on them.
Yeah. Problem is there aren’t any PCIe 4.0 x2 HBAs, though since it’s an x2(16) slot a Broadcom 9500-8i will fit. Lots of drawbacks even if it supports x2, which I’m not sure it does, though.