Resistance to ground on PL6202 was 0Ω. Removed the mosfet and it is shorted on both high side and low side. Fortunately, I have quite a collection of dead Lenovo Legion's, so removed a mosfet from another board. Tested the donor mosfet out of circuit and all was fine. Soldered it to board and the board powers on as soon as I connect the charger. Both APU and GPU get warm.
Re-assembled the laptop, but this time left the battery disconnected. Again, soon as the charger is connected the board powers on. Both the keyboard backlight and fans spin for about 1 second, then go off. Power button stays illuminated. Nothing on the display, no even the backlight. After 30 seconds, the laptop restarts. Keyboard backlight and fans are on for 1 second. This then repeats every 30 seconds. APU and GPU remain warm.
At least the board is now staying powered on without something shorting out, so maybe we are getting somewhere.