Also, for the ball release coil to work reliably it needs a clean earth via the switch 4a on the score motor and make/break switch on the outhole relay (specifically the side that has grey/green wire on one leaf and yellow on the other.)

You can check this earth by leaving the game unplugged,
a) manually move the score motor until switch 4a is closed then
b) put one lead of your meter on the yellow wire side of the outhole leaf switch, and the other lead on earth lug of the ball release coil (the coil under the apron.)

Then operate the outhole relay by hand and look to see you have good continuity and under 1 ohm resistance.

In my EM experience a dirty earth can cause lots of problems .