You picked quite a game to start working on. Sounds like you have a great start. I'm not looking at the manual at the moment but to your question regarding how to work on the playfield, can you get the pcb in a good position to solder by pulling out the playfield and, either laying it out on a table next to the cab or up on its end in front of the machine? If your table/work bench is close enough I don't think you'll have to disconnect the wiring harness. It's pretty long.