OK, so an answer of sorts. Did I mention I was a EE?
Basic debug principles = isolate, isolate, isolate. I started at the Tconn and disconnected both outputs one after the other. I immediately found that only one half the screen was the issue (the left when facing front). This Tcon output feeds two quarter-panel LCD screen drivers in series that have connectors similar to the Tconn. Disconnecting these one at a time, I isolated the issue to just the leftmost quadrant. Disconnect this quadrant and everything works on the remaining 3/4 screen.
Unfortunately.....here I stalled out. There are no further connectors....it's ribbon straight to the glass/panel. I even tried removing the bezel to be sure, but no dice. Tried cleaning with IPA and some mechanical pressure without luck.
It ''could'' be a short on this final driver board and not the panel itself. I ohm's across all the caps as there aren't that many, but nothing obvious. My conclusion is it's the LCD panel itself somehow (I didn't know, or don't have experience, with them failing like this)
The good news is that I can confirm the power-supply, mainboard, two Tconns (original and replacement one I purchased), backlight driver, remote, cable, wifi, etc are all working....so I guess I'll remove all these other parts and sell it on Ebay.