Posting from it: Looks to be a software issue... as on a reboot to Windows 10 (it's a dualbooted arch x64 and 10 x64 setup) I am getting sound on both channels just fine (with a fair bit of intermixing, almost seems to mono but I am getting some left/right separation). Before the jack swap, my headphones didn't seem to plug in right and I had a dead channel on both OS's.
I used a different audio setup on the Arch install on the T460 the succeded this (converted from pulseaudio to pipewire; needed better graphic equalizers to band-stop a gnarly harmonic issue with that particular model)... may try that. Otherwise, I dunno... Depending on what happens with this thing it may end up as Windows 10 only.
I'll upload some pics tomorrow (well, later today technically
)... funny enough despite being footprint and dimensionally compatible, the jacks were different manufacturers and very different construction. Hopefully that's in my favor!