I actually solved the problem this evening, but I can't say for sure what was the culprit. I was being less than intelligent with some alligator clip leads which killed all the backbox lights. I ended up cleaning a couple relays and some switches, but my hunch is it was my sanding/de-oxing of the flat, multi-wire backbox connections to the cabinet circuitry that got it working. I realized I had blown a fuse after all of the work (I'm assuming when the backbox lights went out). After swapping the fuse, the 2-Player Game light was working for the first time ever. Also, the chimes are now triggering for things they weren't triggering on before. This game is loud, almost feel bad for my neighbors playing after 9!
For documentation sake, it was the 2-Player Game light that wasn't working (incremented by the Coin Unit Stepper) and in the case where it should have been lit, the game assumed it was in a 4-Player config (so 1,4,3 or 4 Players). The Player-Up lights were always working in every scenario.
At this point, the only remaining issues I am aware of are:
a) annoying buzzing when "Advances left bonus when lit" lamp is lit (pretty sure replacing the relay for it will solve)
b) 500 relay occasionally gets stuck in a long loop (cleaned relay switches, will monitor)
c) game over and 00/10/20/30, etc. lights have never lit (not sure how to approach these)
Thanks you both for your follow up on this!