Ok, I see why there was a patch wire there. Whoever had a go at repairing this before me lifted one of the very fine traces on the back side of the board under the thin fim resistor network. It was very obvious where this trace was going, so he had just put that wire in there to fix the broken trace. I put a more proper patch wire - (minimum length, more appropriate gauge) - and now we're back to the errors I listed at the start of the thread.
Also, now that I have a chance to put some magnifying goggles on (my vision is failing badly, so I didn't catch this without them earlier), it appears that he's replaced a number of components on the board. It appears he has replaced U153, U103, U105, U106, U104, and possibly Q150, Q151, and C113 (may have just disturbed these in removing the neighboring ICs).