But can they plug in the new Samsung SSD (when available) or any of the PCIe-SSDs currently available on Amazon?

My understanding is that Apple (deliberately) changed the physical shape of the PCIe-SSD connector so you could not upgrade the SSD yourself.

Interesting. EveryMac reports only 2x.

I've seen that OWC article where they get up 1200 MB/s and though that might be still achieved by 2x link width.