First go into switch edge test to see if all of the associated switches register. Make sure you can hear the microswitch click when doing this. You might just need to make adjustments to the wire actuators.

Usually when the game serves two balls, there is a switch issue in the trough or in the shooter lane. The troughs are optos, so you'll have to empty the trough, then drop one ball at a time into the outhole and see if all of the switches register as the ball rolls past all of the optos.

For any associated non-working solenoids, go into solenoid test and cycle the test to make sure all associated solenoids fire. If they do it is probably a switch issue.

When replacing the bulb was the game on or did you just shut it off and possibly short something? The capacitors hold power for a bit longer than you would expect and it is possible to short a switch row or column even with the power off.

Switch edge test. Cant remember offhand which one that is. I edited my post above.

If the power was on, you may have shorted U20, as suspected.

If you pull the main MPU, you might be able to see black marks on the backside. Do you have any experience with soldering/desoldering?

When doing any work under the PF, make sure the power is off, if not already obvious