Ignition Circuit Voltage Drop…where to from here?

Ballast hot is normal.

I forgot "one other place" Does your car still have the WHITE connector in the harness from the firewall to the engine? That might be a factor.

In your measurements, no1 and no3 are probably electrically the same point.

That is, voltage comes out the firewall, and branches off at some point. Without stripping the harness and examining it, I would not know exactly where that is.

Given your measurements, I would bet on ignition switch / connector

Now you say you bypassed the bulkhead, but did you bypass EVERYTHING or only the red and black ammeter circuit? The ignition circuit terminals in there (IGN1) could still be a factor.