So it wasn't the main board at all, turns out it was more power supply problems...
Could have saved myself another 20 bucks, but what the heck I'm still less than $50 into this thing including buying it so I'll call it a win.
I swear this power supply is far more complicated than it needs to be: There's the 5vdc standby supply that's always running (+5vs/1A in the schematic), which powers only a few circuits like the LED on the front and the circuit which tells the rest of it to boot up once you hit the power on switch. Then there's the 12vdc/24vdc end of it that does all the heavy lifting for the inverter and audio sections, but there's another 5vdc leg I didn't see before that powers only the main board (labelled +5v/4A in the schematic) and it wasn't doing anything.
Turns out there's a MOSFET supplying the main board fed from the standby supply, with a pair of transistors telling it turn on with a signal coming from the standby portion of the board. STBY pin is supposed to get latched 1vdc from the power switch, which in turn tells the 12/24v sections to wake up along with the 5v one powering the main board. The 2 NPN transistors and a 4.7k SMD resistor that control the MOSFET were bad (4.7k had drifted down to around 700 ohms because of the shorted transistors), along with another random one in the standby circuit for the 12v leg. That caused all sorts of strange things to happen, like sometimes 12v wouldn't wake up or when I'd jumper out 5v/1A (stby power) to 5v/4A I'd get a bunch of lines on the screen.
The parts on the power supply that were bad are circled in red, and don't forget I had to recap the whole thing as well. I guess if I get another flat screen with a bad supply, I'll save myself the headache and just order a good one. Lesson learned...
