Okay, off work now.

I was using '~' as an 'about', not an electrical symbol.

Tested at work, ~30 secs after starting: Voltage differential between alternator post and battery positive post, 0.48 V, between starter post and battery post, 0.42 V.

*Ding*

Tested playing with the shifter to keep RPMs up - alternator light flickered a little when RPMs dropped low, but no stumbling from the engine or flaring of the other lights. Cranking heater full on, (high and low RPMs) dim flare of alternator light then out again. Turn signals, no alternator light ghost at any speed. Either spinning Fuse 13 worked, or keeping the RPMs up did.

And now for the pay-off, tested at home, 20 minutes of 65 mph hwy driving with lights on later; Alternator post to battery positive: 0.68 V. Starter post to battery post, 0.59 V.

I think I have a prime suspect. The draw of the headlights is helping heat up the big wire and causing what-ever fault is present to magnify to a noticeable degree, and when the alternator drops output with RPMs, it can't push enough power through that resistance until the wire cools again. We'll see if the new cable does anything.