The start up seq sound like it is working normally up to the point that the 100k stepper goes from -1 to zero position. Does the circuit that accomplishes this step (that is, the one to the 100k step up coil that closes via contacts on the 10k stepper when it steps from 90 to 0k) go through the 100k relay (M-relay I assume)? Can you upload this part of the schematic as well as the part that shows the 100k relay circuit?

Otherwise, it sounds like the machine works normally otherwise—I assume this means that when the 10k stepper steps from 90k to 0k that the 100k stepper steps normally and the 100k relay doesn’t stay engaged. Is this the case?

You don’t say whether the 1 point also advances to the zero position?

“The 100,000 relay is energized by the 10,000 unit being in the reset position and a negative one switch on the Points unit (and score motor switch 1A, and a switch on the ball release tray solenoid activator arm).”

I had a similar symptom but for me it was a combo of the points stepper not getting to -1 (gummed up and over-tensioned) and a switch inside the 10,000 unit (had to disassemble to discover the switch) not working.

According to the schematic, the M-relay will stay engaged through completed circuits associated with the following switches: a NC ON ADD 100K sw, a NO S-relay switch, a NO E-relay switch, and a few others associated with the N-relay/10K stepper.
7BEE2529-EFD5-420E-9B7A-C206D863480A (resized).jpeg

Check to see that the NC ON ADD 100K switch is opening when the 100 K stepper steps (it should be on the back of the stepper). If that is OK, then is the associated NO S-relay switch open (S-relay should be disengaged)? I don’t think the associated E-relay and N-relay/10K stepper switches are to blame, but could be worth inspecting too if the NC ONN ADD 100K or NO S-relay switches are fine.

And in terms of the 10K stepper problem, you didn’t upload a photo. It sounds though that your 10K stepper has both right and left hand side switches that are actuated by the actuator arm. Steppers like this that I have seen usually have an extension on the actuator arm that engages switches on one side. I have had a stepper like this in which the extension had broken off the arm and so did what it sounds like you did and moved the switches around so that they could open/close properly when the stepper steps. As long as they’re opening/closing properly, you should be good to go.

Good luck!

Those old games are notorious for having cold solder joints.
Gently yank on a few suspected wires. One or more might just pop off with very little persuasion.