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Check all secondary power rails for voltage pulse and diode mode. Did you test the board alone on the desk?

CD3217 chips must come from same model donor board, maintaining the position also. Compare diode mode readings around all them. They switch to 20V if battery is disconnected? The Magsafe light is controlled by charger, in base of the communication with the MacBook. Do you get some voltages on...

Check in diode mode at least. https://openboarddata.org/?a=showboardsolutions&bpath=laptops/apple/820-00923

Any history available? Check J4401/2 voltages and signals; compare with good board. Sometime need to change U4200.

Did you test with battery connected? Any fault history available?

I mean between capacitors pins, not too ground! You should get 0-2 ohm readings.

You should test with known good trackpad board/flex.

Restarting with just charger connected (no battery)? Check resistance (ohm scale) over C5220/60.

First of all, welcome to the forum! Did you test the machine disconnecting webcam? Post a screenshot from HWMonitor sensors.

SPI_TPAD_EN_CONN goes high for sure?

Did you compare readings with good board? You can also use online refference table. https://openboarddata.org/?a=showboardsolutions&bpath=laptops/apple/820-01521 Check LDO voltages of U3100/200 chips too. Also check diode mode readings on all big coils.

Very likely machine doesn't boot with one dead port. Normally will not switch to 20V on the other port. So compare the readings, as I said. S-UEM2 is very helpfull in this case. I also recommend to test with known good USBC terminals.

"botton port is dead" Possibly broken traces on this port; or bad CC1/2 lines. Compare diode mode readings around U3100/200. The above readings are in diode mode, or voltage?

Be sure JT400 gets all correct voltages (1V85/3V3/5V_S2 and PPBUS_AON). Also check all level shifters from page 84.

"I also noticed something, RIGHT goes high at 1.8V, LEFT goes high at 1.6V" This is not a problem. "Lid closed RIGHT goes low, LEFT stays high" Looks like JR200 is bad; change it. "IPD stays high even when closed" That is caused by JR200; LEFT/RIGHT signals are ORed by UR200.

Using a magnet is the easiest way...

Did you check all three signals (not just two)? You should also check their reaction when close/open the lid.

"Check IPD_LID_OPEN_1V8 and AMR_LEFT/RIGHT_OR_ND_1V8 signals" What about these, once you've got back 1V8_AON?

"I know the keyboard works at least to an extent because again it does respond to the DFU key combination" Wrong assumption! DFU keys are hardware ANDed (also inverted) by UT103. Non working trackpad also blocks the keyboard. Check if it gets all voltages. Try known good trackpad board/flex.

Replace LAS and see if repair assistant can fix the problem.