I 'm late to this party and need to catch up, or try to. Or at least look I'm trying to. If I am repeating anything already discusses, apologies. Frunch is here so you are in good hands.
1) Connector J1 on the the rectifier board is your lights. The read and white are the GI lights. The blues are feature lights and the blue/white is flippers. Try disconnecting J1 to isolate you don't have any shorted lights.
2) Q-17 on the SDU has Orange/Red wire on the J5 connector at pin 7 located at lower right of the SDU. Pull pin #7 from the J5 connector and see what happens. The wire on the other side is a yellow/white wire on J4-11. That is the small 12 pin connector on right hand side of the SDU. So pull the Y/W at pin 11 on connector J4. This should completely take Q-17 and all of its circuitry out of the equation. No promises. See what happens.
3) You mentioned about having to reach all the way over to get to the connectors on the left hand side. All of your wire harnesses are in clusters. Mark your connectors so you know how they go back. Disconnect all the connectors for one cluster of wires and let that cluster drop into the cabinet. Lift your play field, prop it up, and drag that cluster of wires over to the side and re-pin all of the pins you need to re-pin. Then reverse the procedure and reconnect the connectors. It is much easier on the back.
Most of the connectors can only locate to one set of pins on the boards so they are not too hard to figure out how reconnect. However, there are a couple of connectors that you can mis-locate by a couple of pins and plug them to the wrong connecter but if you do that you will be seeing a couple of open pins.
4) I can't find my road map, but each connector is labeled on the boards. J1, J2, J3, J4, and J5. Each connector on the boards in labeled with how many pins are on the connector. 1-12, 1-15, 1-17, etc. The pin numbers can be left to right: 1-12 or right to left 12-1. They can be labeled up to down or down to up. 1 on top 12 on bottom or 12 on top 1 on bottom.
Hope this helps.