looking from the underside of the playfield so you can see the lights, also could try in switch test mode, that Molex plug you mentioned......whilst it's connected try wiggling the wires and see if the lights light up or go off etc.

my thought behind this is I had an issue that drove me round the bend until Quench helped me out by suggesting tracing the wire from the target bank to a Molex plug that whilst it looked intact and fine, but one original wire had possibly been originally been crimped so hard that over time and guessing also much unplugging and connecting led the internal tin plated wires to sever yet stay in the crimp contact due to the outer insulation.
So I'm suggesting checking continuity from the target optos/IDC to the Molex plug, then from the plug to the (not knowing the game) interconnect board IDC?

you could also try reseating the connectors at the interconnect board and the cpu, and the off chance you may have a dry solder join or more on any of the header pins of both boards?

great news!

if it was me and taking the age of the machine into consideration I'd change the opto, but it's just choice.

If I'm right, and that's a big if, I believe the 470 ohm 1/2 watt resistors are on all the time the game is on, being part of the opto circuit R1, R3 & R5, so check to see if you have scorching or cracking under and around them. If so and you are planning on keeping the pin I suggest swapping them out for at least 1 watt ones.....I just put 2W on the opposite side of a similar board and have them about 3/8 off the board so the heat can get away instead of cooking the circuit board.