When setting up a multi-player game, make sure there are at least four credits on the credit wheel if the machine is not set for free play.

If there are enough credits, you'll need to see what sends power to the coin unit step up coil. Check the following:

Coin relay switch - yellow wire and orange/black wire
Score motor switch 3B - orange/black wire and white/brown wire
Reset relay switch - white/brown wire and red/black wire

Make sure these switches are clean and adjusted properly...

For the ball in play issue, you'll need to see what switches send power to the ball count step up coil. This circuit includes the coin unit disc and the player up unit disc. Make sure these two unit discs are operating properly by manually advancing the disc to see if they move smoothly. Check the following switches in the ball count step up coil circuit to see if the switches are clean and adjusted properly:

Outhole relay switch - yellow wire and blue/orange wire
Ball index relay switch - blue/orange wire and red/green wire
Extra ball relay switch - red/green wire and gray wire
Score motor 3A switch - gray wire and gray/red wire

For the spinner issue, check the following:

Clean and adjust the spinner switch if needed -yellow wire and black/white wire
Clean and adjust the spinner relay switch - yellow wire and red/yellow wire

The buzzer is operated by a 6 volt switch on the over-the-top delay relay. Check the switch on the over-the-top delay relay with the white wire and white/red wire. Make sure the switch is clean and adjusted properly. Also, make sure there is a working #455 lamp in the lamp socket of the over-the-top delay relay...

Does this buzzer problem happen with all four player scores? If it does, all four player scores share the same circuit that includes the buzzer. If one of the scores goes to 100K, power will be sent to the over-the-top delay relay coil and to a reset relay switch. Check the following:

Reset relay switch - yellow/black wire and black wire / make sure switch is clean and adjusted properly
Over-the-top delay relay switch - yellow/brown wire and orange wire / make sure switch is clean and adjusted properly
Confirm the over-the-top delay relay coil is working - use a jumper wire or 9-volt battery
Make sure lamp #455 is good at the over-the-top delay relay

This might be the top hole relay. Looking at the solenoid (coil), there is a green/white wire or white/green wire attached to the solenoid. The green/white wire was the only wire I could find that was attached to a solenoid/coil, and it was the top hole relay solenoid. When the ball lands in the top hole, does the bonus advance?

When the bonus advances, does the stepper/relay solenoid in the photo energize?

Maybe they substituted different wire colors on this stepper unit. Now I'm thinking this is a 00-90 match unit. When the game ends, does the backglass show different match numbers after each game? Or does it remain on one match number after every game?

In the photo, there are 11 traces on that stepper/relay unit. This would match up to the bonus unit on the schematic. Are there any problems with adding or collecting the bonus during a game? You can try to manually rotate the stepper unit to see if it changes any feature lights on the playfield or backglass...

You should be able to match the colors of the wires that are attached to the stepper wire traces to the color of the wires on the schematic. This will tell you what the stepper controls...

That looks like a Nickel counter. 10¢ = 5¢x2.

It's shown on your schematic connected with dashed lines (as optional) at position E5. It's described as "2ND COIN UNIT" but in the Bally parts catalog referred to as an Alternator Unit.