Sooo, progress on the Pentium and the Amiga fronts:
While I haven't had the serial mouse I mentioned in the mail yet (it's the weekend after all), by chance I did come across a Logitech PS/2 mouse with its serial adapter still attached, at a flea market on Saturday. The good news here: the mouse works, in both PS/2 and serial mode. The bad news: not on the Pentium 100. CuteMouse and Microsoft's driver detect the mouse and install themselves, but no applications react to mouse movements or clicks, while Logitech's own driver doesn't even detect the mouse. The Logitech driver also contains a simple program that detects activity on the COM ports, from which I can see that the mouse does do something on the P100 when moved/clicked, but that's about it.
Also, which is even worse than the non-working mouse, the P100's power supply seems to be failing. During the two or so hours I tried to get the mouse going yesterday evening, the system shut off without warning three times - complete power loss with the power switch still down (remember, it's AT, not ATX), needed to switch the power off for a bit before it fired up again. One time it even damaged the BIOS settings in CMOS RAM, so that I had to redo the configuration, i.e. HDD parameters, IRQ stuff for the PCI IDE controller, etc. And seeing how I don't have another AT PSU to try and switch this one with, I can't even be sure it is the PSU that's dying... A really frustrating situation.
Next up, the Amiga 500. I had asked a friend of mine earlier this week, if I could have one of the junk A500s he's found at recycling centers over time, basically to have spares for mine. He ended up bringing one over yesterday - A500 Rev 5, 512 KB RAM, Kickstart 1.3, broken FDD (does not recognize disks) - that I could have, in exchange for someday setting up an old PC of his, where he's planning to replace the HDD with a CF card in an adapter, with Windows 95. Some of that A500's parts already found their way into my main system, including the keyboard's keycaps, the RF shielding and some screws, which all were in better condition on that one.
And speaking of the Amiga, at the same flea market, and the same stall even, where I found the serial mouse for the Pentium, we had also found a bag - literally - of seven(!) Competition Pro joysticks for ex. the Amiga, C64, and anything else with a compatible joystick port - five normal-sized (three of them black, two transparent blue) and two half-sized (one transparent white, the other transparent blue), for 0.50 € each. The downside of that: only two of them are working properly right now, while the others have non-working directions and/or fire buttons. All of them do something, so we'll be trying to diagnose and repair them next weekend.
Edit: We have three working Competition Pro joysticks; the second transparent blue one is now working fine as well. The right directional button apparently was stuck or something, because after some (not so) gentle wiggling around, that direction is back in action.
Edit #2: The PC mouse problem has been solved. Turns out the IRQ settings for the COM ports were swapped on the multi I/O card! COM1 is supposed to use IRQ4, while COM2 is supposed to use IRQ3. Switched those around, bam! The Logitech drivers for DOS and Windows installed fine and work perfectly. Now to diagnose those random power losses, although I haven't had one today so far. Also, Doom's attract mode has been looping for the last 20 or 25 minutes, without any hickups, so... I guess the PSU might still be fine? But I probably should get myself a multimeter soon anyway, to check the voltages it's outputting.