Turns out I swapped the core from one broken PBC to another broken PCB. No other option than to try and repair this now. But the faults is tricky to find, and there are few of them! Some luck required.
To summarise the faults on this "good PCB": - 5V regulator not working - 12V leaking to 3.3V when 5V is enabled and running - HDMI port needs replacing - PEX buck replaced - Shorted PWM signal in one of the fans
