JNL0122pressing the power switch after it is up and running does power down the unit properly, it just will not start the unit.
So first, this is a good way to check everything else (like power supply). Most parts can't be bad if it's running.
This behavior would seem to indicate the actual soldered in switch is working (proper continuity across contacts) but still a good idea to replace this whole "power switch board".
Sounds like something is shorting or holding machine down. Sounds like a power PG-ms problem.
I wouldn't think an aux board with USB etc. could hold machine down. You can usually just disconnect stuff like this and the machine will run fine without it (you just can't use those ports, obviously).
Can you make machine any more minimal or basic to acquire a good startup?