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G74sx not booting suddenly. Unusual hardware problem.

Level 7

I've run into an unusual problem - waking up Jan 1st my laptop didn't boot. Power button lights up, but nothing else happens but fan spin and some indicator lights. Both CPU and GPU get warm to the touch. The LED for charging lights up only if the battery is connected, and the LED indicator for power is on (left of the battery charge indicator), as is the 2 indicators above the keyboard to the left (one is for caps lock, cant remember the other). No light in the last 2 indicator LEDs (drive activity and bluetooth) on the bottom left.
Considering all those LEDs are on (2 separate) daughter boards I assume those work fine.

Figured the motherboard was fried or something, so I tried to get it diagnosed by a local tech. Nothing he could do without going on with board level diagnostics (more expensive than the laptop itself). Ordered a new motherboard, installed it (reused the CPU and heat sink with new thermal paste, and the GPU is new since it is soldered onto the motherboard which was replaced). But even without either it should give some sort of noise when trying to boot, correct? 

I have taken out the 3 HDDs and ram sticks which when tested all function perfectly fine, so I've ruled them out as the culprit. I'm not an electronics engineer, but to my knowledge a working motherboard should give some sort of indication when hooked up to power, yes? And yes, I did replace the CMOS battery. A thing to note is that the laptop has been running with only 1 of the 2 fans for about 4 years at this point, but I have been cleaning it out regularly and monitoring temps, so considering they are right next to each other it doesn't seem to have been a problem and as mentioned both GPU and CPU get warm to the touch, so there is some level of activity there.

About the screen - it doesn't seem to be a problem, and as far as I can tell plenty of people have busted screens and use an external monitor instead. I've tested booting with 3 different monitors (through HDMI), and no signal comes through apparently.

Only thing I can figure is that the power button is on a small daughter board that might be misbehaving in some way? Don't know the schematics/board view so hard to tell. Unknown if the ROG-button next to the power button works, since it only has a function inside windows.

So, a new motherboard, new GPU, CPU seems fine, working RAM, working HDDs and no response to power on whatsoever (charging brick and battery are just a few months old).

Anyone have an idea what might be wrong? Ribbon cables? Neighbors black magic? I have little to no experience with boards that have the power button on a daughter board so I'm not sure how the signaling runs and how it would react if something was faulty there. Any help would truly be appreciated.