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Just for test, replace LT400 with a jumper. Be sure all LT20x from page 85 are good. Check if you get corresct power rails on JT201.

You should check DCIN/BMON_ISENSE instead. Their voltage depends by the current consumption from the charger and battery charging. 2V looks too low for PBUS_VSNS_OUT; it should be close to 12.5V. You should get 1.33V aprox at PBUS_VSENSE.

What is exact 820-xxxxx code of your board? What damaged component found on the mainboard?

Use ticker cables to inject voltage. You must heat the board, not the cables. Also use powerfull lab PSU and set amp limit to max first. Then start to inject with 1V; increase, if needed. If PSU gets in limitation mode, may not inject anything. If it supports CC mode, that helps.

"no caps lock LED working" You should follow KBD_CAP_CATHODE trace; driven by UT105. Seems that machine restarts only when more CPU cores are engaged. So you may have a problem with CPU itself. I recommend to inspect all I/V sensor circuitries, from pages 59-64.

You should do any other check possible before reflowing the chips. If you suspect GPU chip, you can lift (one side) all LA710-40 coils. In case if Intel side is OK, board should stay on, getting even USB activity.

Still looping if LCD is disconnected? Test the board alone on the desk. Check diode mode to ground on all big coils. Compare readings with good board.

Inject why??? You don't have short there, nor even low resistance. Just in case, replace F7140.

First of all, welcome to the forum! What is the exact ohm value of the short? How are you sure that actually inject 10W there?

If you can complete DFU revive, there may be a chance to save data, if needed.

Just need to fix U9500 problem and replace the NANDs.

Any history available? Tested w/ and w/o battery connected?

Try to access DFU mode, using external switch method. https://logi.wiki/index.php/DFU_Mode_Restore_(Macs)#DFU_Mode_using_external_switch

You have all MPMU LDOs on page 37. Check the input voltages on left side and LDOs on right side. Do the same for SPMU, page 32.

All other M/SPMU LDOs are present? If isn't recognized in DFU, you may have problems with CPU. Just in case, compare diode mode readings around UF400/500 with good board. Check their LDOs with charger alone.

There are more LDOs available. You may have just a small capacitor, or resistor corroded somewhere. But also is possible to have more damage; maybe corrosion under some BGAs. Is the machine recognized in DFU mode?

Check diode mode to ground on all secondary voltages. Test the board alone on the desk. Is the machine recognized in DFU mode?

You've said the connection board was also changed. Where is the short then? Post its exact resistance to ground, in ohm scale.

Missing enable signal; not even pulsing? EDP_PANEL_PWR_EN is present?

Test in safe mode, with charger and battery connected. Check CHGR_A/BMON levels.