Re: AFCI woes
I'd look into seeing if there are multigang boxes housing switches controlling loads on other circuits, that's quick.
Then I'd resort to:
opening the panel,
locating my AFCI circuit,
shut it off,
remove the AFCI load neutral,
wirenut a test lead of my tester to it, testing audible continuity,
touch the other lead to the neutral bar to verify the bootleg neutral,
shut off other circuits,
remove their neutrals one at a time,
testing for the source of the bootleg neutral.
Once you've identified the circuit(s, don't stop when you find one), you can narrow it down further. If you don't find one, you've got a neutral to ground (if available).
Or something like that.

I've got one I'm avoiding troubleshooting right now myself, they're always a major headache.
