So, after taking a break from this thing, I successfully fixed a missing mouse problem on my Pentium MMX (233)
It was apparently due to serial (COM) port header, which is quite odd, since I previously replaced it for another one. Seriously, what are the odds of having TWO mouse headers not working properly?!
On a side note, I did get some more problems from the replacement VIA board, as I first couldn't find the drivers which would work under W95, and then once I finally got them & installed everything, the hard drive started acting up. It wouldn't read data, sometimes it wouldn't even boot up Windows, so I replaced it for another, higher capacity (10GB) model... As if that wasn't enough, my CD-R drive started doing exactly the same, except it wasn't happening during the POST or boot process, it would happen within the Windows session.
Even though I was pretty sure this one is NOT hardware related, I swapped the CD-R drive for older (and therefore more authentic) 24X Teac CD-ROM unit. The old CD-RW drive was mainly in good shape, but the headphones volume control wheel got stuck at some point, I'm guessing the potentiometer inside either broke or something fell inside it, so to be on a safe side I scrapped the entire unit. There's plenty more of those around, identical to the one
@Robert B showed in one the latest posts, so it's not worth it. It also occurred me at some point that the hard drive fault(s) might had also been related to software issues (enabling DMA feature in particular), but since the hard drive would also stop working in DOS or even within FDISK app, during the partition editing procedure (it would report "disk write protected") I'm pretty sure this other one is hardware. However, with all these failing components inside, I'm wondering if the old motherboard really WAS damaged to begin with, or perhaps was it just terribly "confused" due to bad COM headers & failing hard drives...? Good thing I still have it, so I might give it a quick test.