A: Physically yes, but it will run at Gen4 speeds (~7 GB/s). The card's controller will still use Gen5 equalization, but the link trains at Gen4.
If you are a hardware engineer, PCB designer, or serious enthusiast builder, obtaining and studying this document is non-negotiable. It will save you from failed link training, corrupted data due to crosstalk, and overheated drives. For the rest of us, understanding that such a specification exists helps explain why your next M.2 SSD might cost more, run hotter, and demand a motherboard designed with military-grade trace routing. pci express m.2 specification revision 5.0 version 1.0 pdf
A: This is often a signal integrity issue. The motherboard likely does not fully meet the M.2 Rev 5.0 spec (e.g., trace length too long). Check for BIOS updates that tighten signal settings. A: Physically yes, but it will run at Gen4 speeds (~7 GB/s)