Interesting measurements.
In your second post that updated the statement of the failure, you said that you had the external device connected to Vcc (5V), not Vdd (3.3V).? Is that correct? Or was it the 3.3V rail?? or both?? The answer to this definitely guides the troubleshooting process.
Being on the 5V Vcc supply is why I said you should check Q108 and Q105, since a back-voltage on the 5V rail should hopefully only affect this part of the circuit.
And your test shows that Q108 is likely blown, which would prevent the 5V Vcc supply from coming up.? The failure could also have damaged other components in that same part of the schematic, to the right of Q105.? Only tests/measurements could say for sure.? If there is a blown component that shorts to ground (e.g. D106), it could cause significant current draw on the 12V line.
But your tests also show something holding down the output of the 3.3V Vdd supply.? So obviously something is wrong here, too.? The only thing connecting across these two supplies is the 12V line, and it is getting enough voltage to power IC101.? Note that normally, the IC101 3.3V is only 'on' for about 1/4 second right after power-on, while the firmware boots and enables the more efficient 3.3V supply.? This is obviously not happening, so the mcu is never getting enough voltage to boot.? I'm not sure exactly what to look at next, but with the output of IC101 lifted, and power_on set, I would check the voltage on the 12V line, and at the input and output of Q109, Q111, Q108, and test D103.? That will give some additional data points to guide the debugging...? Good luck.
