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It should have a separation fuse for PPBUS_AON. "The voltage at the PPBUS_AON diode is 0.012V" I'm not sure what this means. I always said NEVER write end "V" for diode mode readings; just to avoid a confusion between voltage and diode readings. If you really have 0.012 in diode mode, that is...

Guided by A2681 (02536), disconnect F5200 and check C5252 voltage.

Check big coils voltage with board alone on the desk; nothing connected, apart of charger.

Which schematic are you using? "PPBUS_AON line has an abnormal value of 0.075V" If that is voltage, what is diode mode to ground reading? Remove battery + charger and wait 15s before checking.

Is the machine recognized in DFU?

Do you get voltage on any coil before restarts? Also check U7800 LDOs, G3S and SSD voltages.

Many coils you didn't check there. Anyway as RAM voltage is missing, the board doesn't boot. Any fault history available? It is recognized in DFU?

Be sure to use 96/140W charger. Also test with charged battery.

Stuck on apple logo, or goes black/off?

Possibly more liquid damage on secondary power rails, or other sensors. What about M/SPMU LDOs? Compare diode mode readings with good board. Did you test with charged battery?

Both R8450/1 have correct ohm value? Check U8450 output voltage also.

"IPD_SYS_AWAKE is present *without* trackpad connected" In such case, CPU is the one which first wakes up the trackpad. Seems that even you force on the trackpad, CPU doesn't talk with it. I suppose that CPU can't start such communication, without sending AWAKE signal by itself.

So board reacted at heat applied on the NANDs replacement process. I still suspect some leakage caused by corrosion somewhere.

"I ran a wire from pin 1 to pin 3" Pin 4 went high then? Check IPD_LID_OPEN_1V8 and IPD_WAKE_CONN_L; compare with good board. BTW, check JT400 signals when trackpad is connected. I hope its power rails are all good.

Then something else is affected by corrosion. Can be just one resistor, or capacitor; not easy to find. BGA chips are suspect also.

Did you test with known good (2015 year) screen?

UG400 is directly connected to 1V2_S2. What is IPD_SYS_AWAKE voltage level on working board?

Mentioned capacitor didn't kill anything; it failed because of overvoltage on 1V2_S2 power rail, like all other chips. You should try several times DFU restore. If trackpad worked before attempting the restore, there should't be any hardware problem.

Check AUDIO_JACK_TIP/RING_SNS levels when connect/disconnect headphones.

You should test known good audio jack also.