If it plays 3 ball okay with > 1 player doesn't that indicate the jones plug connection is okay, since that last ball function on the schematic is determined by that single plug wire? There are probably other pins on the jones plug but they would alter certain game features only.

Either contact rivet at position 8 on the player unit (at right of the red X) could have a broken wire at the wiring lug, or is not making contact with the wiper fingers due to wear/crud on the contact, or its wiper finger is dry/dirty/sticking.