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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.

Check UC120 voltages.

Install macOS Tahoe (if possible) and try to recalibrate LAS.

That points to CPU graphics problem. Seems that was affected by voltage spikes, when removing video cable (with power connected).

Check IPD_LID_OPEN_1V8 and AMR_LEFT/RIGHT_OR_ND_1V8 signals.

You should always get correct external image when internal LCD is disconnected. If not, CPU is suspect.

So doesn't turn on at all with charger alone? Check UPC_I2C_INT_L and UPC_SMC_I2C_INT_L levels.

What said the customer about fault appeared? Did you test with charger alone (96W)?

99.9%...

Short on two different power rails, with NANDs as common point, guess what?

Do you note any short on the power rails of these chips?