Hi, Scippylisp.

About the screw of PCI bus number.
There may be no problem if the numbers are not overlapped each other.
These are automatically numbered, it may be a consecutive number or not be so.

Physical Slot2(PCIEx16) of this MOBO is different from other PCIEx1.
It seems that it is directly connected to the CPU.

You can find it under "Intel (R) Xeon (R) processor E3-1200 v3 / 4th Gen Core processor PCI Express x16 Controller".
Not "Intel (R) 8 Series / C220 Series PCI Express Root Port".

However,
It seems that "the behavior of PCIE assignment to memory" was surely changed in version 1709.
Combining version 1709 with "this MOBO with less memory" seems to make the problem serious.

By the way,
6GPUs on my rig running with windows10 1709 now.

OS: Windows10pro 64bit 1709 (Clean install from USB media. Then edit HackFlags turn on)
MB: ASrock H81 BTC Pro R2.0 (BIOS v1.20, Setting is "load default".)
CPU: Intel Celeron G1840
RAM: DDR3 8GBx2 (16GB)
GPUs
 PCIE1: GTX1060 6GB
 PCIE2: GTX1080
 PCIE3: GTX1070
 PCIE4: GTX1070
 PCIE5: GTX1070
 PCIE6: GTX1070

(All GPUs connected via riser card. and Disabling HD audio devices in device manager.)

When turn off HackFlags and enable audio devices,  Three GPUs indicate code12.