On the ball index relay, find a normally open switch with yellow & grey-yellow wires connected. Check that the switch is not closed or wire connections touching. Adjust switch if needed.

The 2nd grey-yellow wire connects to a 3 normally closed switch daisy chain circuit on the 3 scoring relays in the backbox. That’s why the switch reads to be closed.
With the machine reset, ball removed, playfield lifted and supported. Manually engage the extra ball relay. The first points scored should release the extra ball relay hold circuit and release the extra ball relay instead of pulling the ball index relay back in.
If the ball index relay is pulling back in, then check on the extra ball relay for a normally closed switch with br-g & grey-blue wires connected. Check that this switch is opening properly when the relay pulls in. Check for shorted wire connections.

In post #3 , repeat the resistance test on the ball index relay switch with the yellow & grey-yellow wires. The meter should show a complete circuit.
Have someone individually manually push on the backbox scoring relays. When each relay is pushed, the closed circuit meter reading should disappear. Is this what happens when each relay is manually pushed in?

Make adjustments to these switches:
On the extra ball relay, the normally closed switch with the br-g wire & grey-blue wire.
Adjust switch to open early and reclose late.
The normally open hold switch with grey-yellow & y-b wires. Adjust this switch to close late and open early.
The hold switch needs to be adjusted so that it opens long before the normally closed switch recloses.
The 3rd switch to adjust is on the ball index relay. A normally open switch with yellow & grey-yellow wires. Adjust the switch to close late and open early.
Retest extra ball release.

On the bonus unit wiper disc, the wiper blade in the 6:00 o’clock position when the unit is at zero position is the power feed for the ball index relay.
Slip a piece of paper in between the blade and rivets. This should keep the ball index relay from pulling in.
Repeat the extra ball release test.
Does the ball index relay still pull in with the wiper finger blocked?
Does the extra ball relay release correctly when 1st points are scored on the playfield for each of the 3 backbox scoring relays?

On the tilt relay, find the normally open switch with yellow & grey-blue wires. If this switch is closed or connections shorted, then this is the alternate path to close the ball index relay.
Clean and inspect switch for correct gap. Adjust if needed.
Inspect ball index relay operation.
Retest extra ball release.

Ok it’s a little clearer now. With the machine reset and playfield lifted:
Manually push in ball index relay.
Unplug score motor service jack.
Put a slip of paper in score motor cam 4 switch A.
Manually push in outhole relay.
Manually turn cams until cam 4 drops it’s switch stack.
The normally closed part of switch C on cam 4 and a normally closed switch on the outhole will both now be open to allow the ball index relay to release. Unless both switches are able to open the ball index relay will not release its hold circuit.
Turning the cams to cam 5 should release the outhole relay hold circuit.
Inspect and adjust switches if needed.
Plug service jack pin back in and retest.