OK, I went back & put the instrument custer back in and took some more measurements.
I thought that the wire masurement was a bit high. 5ft of 12AWG wire should be about 8mOhms per the tables. I pulled the contact out of the regulator harness side connector, cleaned it up and remeasured to 13mOhms. That included crimped connections, so a reasonable number.
The meaurement of the in-place ammeter circuit path was ~0.875 Ohms.
From the junction block to the same regulator contact as above.
the circuit simplified looks like:
That drawing should say alternator out, not voltage regulator out, I think that is the equivalent of VR in sense.
Putting a low value resistor in series with the battery charging cable will slightly increase the voltage drop on across that segment & move the needle more significantly. The other way to look at it is, for a fixed current to/from the battery, increasing the resistance slightly in the 12AWG red charging wire path diverts a larger portion of that total fixed current through the parallel gauge path. Restrict flow in one of two paths of a river and what is needed simply goes to the other branch. The total flow won't decrease.