Cannot get Windows 7 to install

You said:

"If the USB mouse/keyboard is supported when operating in the BIOS, then obviously the firmware of the motherboard has that capability, why cannot that capability not then also be used during any installation process ?"

I am assuming you mean clean installing Windows 7 then upgrading to Windows 10. I believe you are right that the firmware can read mouse and keyboard inputs via either USB or PS/2. However, inside the Windows, drivers are needed to communicate with hardware. As Xaltar stated above, all USB communication, whether USB 2.0 or USB 3.0, goes through the USB 3.0 circuitry on the motherboard. AS Windows 7 does not natively have drivers for this circuitry, it cannot communicate with any USB peripherals. If you could get the AMD chipset drivers from here:

https://www.amd.com/en/support/chipsets/amd-socket-am4/a320

and install them, then your USB keyboard and mouse would operate. As you stated you do have PS/2 keyboard and mouse, this adapter might work, but I am not sure:

https://www.newegg.com/beige-startech-6-others/p/N82E16812200078

Perhaps someone here might know, or opening a ticket with Asrock support might clear this up. Sorry for being long winded.

Jake