For the credit button issue, when you press the credit button with credits showing on the credit unit, does the credit relay coil energize? When you press the credit button, can you hear a relay clicking?

For the ball in play lights, check the game over relay. The switch with the brown wire and gray/red wire need to be closed after the game resets to ball 1... Make sure this switch is clean and adjusted properly.

After the 5th ball drains into the outhole, is the 5th ball light in the backglass still lit? Make sure the ball count unit is advancing fully to the 6th position contact rivet after the 5th ball drains into the outhole. The contact wiper blade needs to be centered over the 6th position contact rivet. Make sure the the 6th position contact rivet is clean.

If the ball count unit advances to the 6th position, does the game over relay coil with the green/red wire energize and trip? If the game over relay coil does not energize and trip, you can use a jumper wire between the ball count unit on the 7th position wire lug (green/red wire) and the game over relay coil (green/red wire) to test the circuit.

If the game over relay and the ball count unit are in different locations in the machine - e.g. game over relay in main cabinet / ball count unit in backbox - then it's probably a Jones plug issue. Make sure all the Jones plugs are clean and seated tightly.

If the game over relay and ball count unit are in the same location - e.g. game over relay and ball count unit in main cabinet - you would have to trace the green/red wire to see if it's broken or there could be a faulty solder joint at the connection points.

There should be just one green/red wire between the game over relay and the ball count unit. You'll probably need to un-bundle the wiring harness to identify that single green/red wire. There might be an internal break in the wire, or possible faulty solder joint at the wire lugs.