Look down at the transformer in the cabinet. There's a connector there with a bunch of YEL and WHT-YEL wires. That connector might have some evidence of prior excessive heat. If not you can check VAC at that connector to try to isolate where you're losing voltage.

You measure AC by connecting on lead to one wire and the other to the other wire. The reference is between the two points.

You (typically) measure DC by connecting the black lead to ground and the red lead to the point you're testing DC voltage.

I can't remember if one side of the System 11 GI is tied to ground but for WPC one side of the GI is definitely tied to ground on the PDB. The advice you were given above is not generally valid for AC.

You can prove that the header and/or trace is the problem by installing jumper wires. I try to figure out the cause of the problem. If after installing the new board your issue still exists the new board hasn't solved the problem. You still have the problem to deal with.

On the other hand you have a brand spanking shiny new board in the machine that will last for years to come.

Just follow whatever the schematic says. Usually (not always) the schematics in the Williams manuals are correct. I think there might be a thread on here with documented errors in the schematics. The interconnect board is fairly simple though. It's mostly pass-through.