Just to follow up my own post, this bass is working properly now. It turns out the mystery push-pull pot was not a push pull pot at all ...it was merely a broken pot!
Here's a full explanation of the knob functions - (Pardon the lousy playing, I'm a keyboard guy): https://www.youtube.com/watch?v=fpjKFYOARDk
If anybody else working on one of these is using the B30 schematic on the vintage washburn site, it seems to be pretty close, but it is definitely not the same as this guitar, or the other B30 I've seen. The pots are different values and the guitars do not have any switches; also, there are fewer wires from the pickups. I *think* the schematic is for a different pickup which could also run in passive mode. But it was close enough, anyhow, to help me track down the micro-crack in the PCB I had to repair.