Thanks for your reply, arjepsen. I hope you're wrong about my CPU/PSU's viability, too!
Latest developments/thoughts in this saga:
1) I found some RAM to upgrade to OS v5.1 (uncracked), on CF card.
The Database info is INCORRECT re 256MB RAM DIMMs not working in Amptron MB; I used two PC133 256MB DIMMs (16-chip, so apparently it's older PC133 that's backwards compatible with PC100 with 16MB-max chips that are necessary). Amptron recognizes 256MB total (not sure if via one entire DIMM or else seeing only halved 128MB on each of the two).
(DIMMs barely fit, I gave up three times before forcing them in controlled manner, despite shallower apparently narrower slots in Amptron compared to old Dell PC MB they came from.)
Despite v5.1, the keyboard & mouse are still unresponsive.
2) The Apple aluminum USB keyboard that user RJH_MUSIC successfully used on Itox MB is only seen by Amptron as a USB "Hub" and not a keyboard (indeed it has its own two USB ports), and cannot even be used for BIOS settings on Amptron MB.
3) I've determined that my CPU/PSU keyboard & mouse ports have no physical damage or excessive wear (like might be more common on a normal PC, since different form factor/usage of Mackie PSU/CPU) so issue is very likely something other than the port connectors or soldering being bad, and I'll rule that out for now.
4) Amptron MB has two ITE chips (IT8679F-A "Super I/O" and IT8687R "I/O Buffer") that apparently control all I/O ports including keyboard, mouse, Console Data port (which clearly works), floppy drive (works), and also possibly USB ports (which respond properly in BIOS/startup), so it's not anything like a bad "keyboard chip" some others on forum have claimed. So what is the real issue? probably something that is common to both keyboard and mouse ports, that are physically close togehter. Maybe pull-up resistors? What are the possibilities, and how do I test?
5) How can I test keyboard and mouse ports using multimeter and sewing needle (according to some online opinions), at least for power? Need exact detailed procedure.
6) Should I wire in another PS/2 mouse port to Amptron's "J4" separate/second/internal PS/2 interface header pins? Maybe it will work.
7) The two Amptron USB BIOS settings need explanation, which is very strangely competely absent from user manual. Why MUST you set ONLY "OnBoard USB Function" to ENABLED but leave "USB Function Support" DISABLED or else the SYSTEM HANGS with a blinking cursor at one of the boot screens when you reboot with the new BIOS settings?
8) Will flashing/updating Amptron/AmiBIOS BIOS from my current 5-19-1998 version to the apparently newest 11-27-1998 possibly fix and/or add USB options, that may in turn work in D8B OS? On the other hand, is flashing new BIOS version too risky, possibly bricking MB, or losing D8B OS compatibility? Have any Amptron DB8 owners received the 11-27-1998 BIOS by default on MB from Mackie?
That's all for now; comments and suggestions are very welcomed!