After spending many hours studying schematics, I resolved to give the basics another go. I cleaned the player-unit P5 contacts. Again.
I pressed the button, watched the dance, and OMG the AX relay unlatched! Reset sequence completed! It was actually the player unit not signalling home position blocking it, despite having attempted to clean the contacts previously.
But then, new problems: I turned the machine off and on again, and suddenly the player unit wasn't stepping. Spent a lot of time studying the circuitry for that, but in the end I managed to fix things via manually stepping through it. Depressing the solenoid, I got through each step manually. Everything behaved as expected, for each step, except for the fact the player unit wasn't manually moving forward. Got to to the end. Reset the game, and hmmm suddenly it's stepping fine.
Was something stuck in it's sequence? I am not sure what it could have been, but I expect this issue might rear it's head again in the near future.
So after that I dropped the playfield back in, put the ball in the drain hole, added credits, and then pressed the start button. Player unit clacks through, everything resets OK, AX motor unlatches properly, stopping the player motor, and the ball is ejected in to the shooter lane! Ahhhh it's working!
But then, uh oh... The flippers aren't flipping. The playfield is lit, but nothing is activating on the playfield.
I check the schematics: There seems to be a Q / AX combo that disables the upper playfield, but the schematics don't actually show ANY normal path for the circuit to return to ground. I am a bit confused by this.
I tried chopsticking the Q relay's NC contacts, but all seems nominal there, so what could it be? AX is latched. Q (game over relay) is off, as expected, the NC connections all seem good. The schematic itself seems ambiguous as to how the proper circuit completes to allow the basic flipper operation.
That's where it stands as of now. More investigation tomorrow.