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Remove U7000 and check again. If still short, then bad C7021/2; which I doubt. If short gone, replace U7000.

"There may be a communication problem between PCH, CPU and BIOS" Check all SPI BUs Series Termination resistors and all their traces/solders. Try flashing known good BIOS. Check all PCH's voltages from page 22; there may be open series resistor, or coil. Last options, reflow PCH and CPU; to...

I don't see any resistor connected to pin 17; there is only one capacitor (C7273). It MUST have 5V; if not, replace U7200.

I mean ohm value; without power, of course.

Check PMU_VDDHI behavior. Also check diode mode to ground on U8100's missing LDOs; disconnect any power first.

Low Drop Output (LDO) rails are clearly marked on pages 42 (Master PMU) & 38 (Slave PMU). BTW, don't bother with the ones marked "NC" (Not Connected)!

Not easy to diagnose the MCP. It may cause weird issues.

Check directly between pins 27/28 of U7000; should get the same value.

"17- 13.4v" I really doubt about this. Be sure do not touch pins 16/17 at same time! Check diode mode to ground at pin 8.

"Could the 0 volts on L7550 be indicative of an issue?" I already mentioned on other post, this board doesn't get iGPU core voltage on boot! It is enabled only when hard graphics load is detected.

Try replacing U7000. Be sure first, no short to ground on pins 19, or 20.

Compare diode mode readings on secondary power rails with good board. You can also use reference table: https://openboarddata.org/?a=showboardsolutions&bpath=laptops/apple/820-01958 Test if boots with a battery holding some charge.

Remove CPU heatsink first.

Any history available? What about 3V8_AON and U8100's LDOs?

Possibly problems with LCD, or its connector.

CPU core should drop from 1.1V to 0.7V aprox, when board boots. There may be a communication problem between PCH, CPU and BIOS. BTW, stop checking DC voltage on any coil's input pin.

Post U7200 basic voltages: pins 16, 3, 8, 17, 1, 6.

Post U7000 basic voltages: pins 1, 2, 3, 9, 12, 13, 14, 15, 17, 19, 20, 27.

"SMC_DCIN_ISENSE- 0.73v" This looks very high! Check resistance (ohm scale) over C7020. R7021/2 are the first suspects.

MCP also fails after so many years. Refllow is not a solution for these old boards. You can probably find working one very cheap.