As promised, I am 99.99% sure I worked out the line-level audio issue with the A board.I'll take a peak tonight, overnight and get answers on all that in the morning.
That one trace being cut, seems like a small and easily repairable sacrifice to leave that pot in place, so I agree.
No trace cutting, unless you want to

The OG volume pot stays in place as is
and you will have sound through the jamma harness and the rca jacks.
When you can, if you can verify those few questions regarding the cases I would appreciate it.