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It seems you haven't worked on a 820-00165 yet... these boards don't "Fan spins 3 times" like a 820-3437 would. Only once.
Plug a battery and check for Vcore again.
If no vcore, check resistance to ground on vcore and report.
Do you get the screen to light up at least when the span is spinning?
And we still don't know if CPU Vcore is present. I bet it is as they usually do not power cycle otherwise. If CPU Vcore is present and this was a out of the blue failure the only thing I can advice is to try a known good BIOS.