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Remove C63 and recheck the voltage divider output. If it's good, then C63 is bad, if it's still low then the MCU is suspect.
So, every vehicle with a running engine can drive too then? The only place that voltage divider goes is the MCU, hence if it's being loaded down the issue is most likely in the MCU.
No idea what the specifics of it all are, though once the battery is plugged in I'm sure there is some back and forth between the MCU and PMIC. The 3.2v rail is on all the time though, so it can tell if you have pressed the PS button and provided the battery voltage is high enough and the MCU can tell what it is.
Option button has a small pad just under it that could be used, easier to just pry the top off it and solder to the center contact though. Similar for the Share button, but it's little pad is on the other side of the board, again just easier to remove the top of the switch. For the TP, just wire to the bottom left lead of it. All of those buttons use Ground for the common. If you are not going to have the FPC (Flexible Printed Circuit) connected to the main board, you need to install a couple of Resistors on the main board to 'mimic' that it is on there, or the L2/R2 will act weird and cause odd issues with the controller.