It's definitely possible see https://www.nicehash.com/index.jsp?p=news&id=155

One of the M.2 is pcie only and the other can be pcie or sata is why only one has the option.  I don't know why the 8th card causes a problem.  It does sound like a pcie sharing problem so maybe more tinkering in the bios necessary.  I assume your total power supply power is at least 1500?