Amongst the repairs you mentioned in your first post you replaced both PIA chips. Did you prove whether either of the original PIAs were faulty?
What frequencies are you measuring for the display and zero crossing interrupts at pin 40 of U11 and pin 18 of U10 respectively?

If the board ever had any battery corrosion, post some clear pictures so we can see how extensive it became.

So switches on return lines I5, I6 and I7 are generally working. At the J2 pin header on the MPU board, what happens if you directly jumper a wire from any switch strobe line (say strobe #0 at pin 1) to any switch return lines: I0, I1, I2, I3 and I4 (pins 8, 9, 10, 11 and 12 respectively)? Does switch test mode register anything?

Cheers, yeah if you can take pictures outdoors when it's daylight (but in shade so there's no shadows) they'll come up much clearer. Those preliminary pics show the board generally looks in decent shape.