I do almost everything in the box to save space, so I don't have much experience with old synths. What old hardware I have that I use is mono. Actually I use the MPE side of Touchkeys pretty rarely, it's mostly for on key modwheel/pitch bend access with mono synths external/internal. I remember hearing about an old synth that actually didn't have poly aftertouch on its keys but could support it via midi, forget what it was, I wonder how often that happened? Otherwise changing a channel AT synth into a poly, well if it was multitimbral, or you assigned each voice to a different midi channel then split the keyboard, you could at least have channel AT for each split.
Regarding ethernet problems, if there was some sort of com port/ethernet conflict I'd try an add on ethernet usb stick, aiming for something different chip/driver-wise than what came with the motherboard if I couldn't fix it with driver updates/software/bios fiddling. I had usb troubles when I started out because my USB ports weren't providing enough power, I tried the special cable which fixes it for some people but not me, what actually fixed it in my case was changing the USB setup in my bios, but that caused other issues. Later on I got a different brand motherboard with different USB drivers, and no longer had any issues at all.