Yes PFC (slip of the digit)
See if Vcc rises high enough without the PFC connected first
For applying external 12V supply also need to bias LINE at 1.24-6V which could be done with a temporary 33k resistor from Vcc to LINE (should set ~4V). Don't forget to remove it before replacing the fuse :O
with no HV should be able to safely poke around with instruments on both sides of the IC to get measurements
It's hard to put all this in text but I'd leave the mains completely disconnected while testing with external supply (not just via F101) so there's nothing on the board that can bite you or your tools.
EDIT: IF there's 16V on D510's cathode then it would appear the major voltage drop is across Q501 C-E (5V or more) so it's not fully switching on but it IS switching from earlier probing. Check that R508 (1k) hasn't gone high and get a voltage reading on the base of Q501 with opto IC503 pins 3 & 4 shorted.
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