Alright, definitely found an issue with the fuel sender. I cut the wire at the tach, tested continuity and it was open from ground to the pink wire connected to the tach, and shorted to ground on the fuel sender side.
Put a fuse back in #3, turned the ignition to "ON". No ignition light (just dawned on me that I don't have a light because the "I" terminal of the voltage regulator is still disconnected. Will remedy that an report back). Checked the fuse, it's still intact. Started the engine. Charges normally. Turning on the fan causes the ammeter to drop as it should.
That's the good news. Now for the rest of the mystery.
After installing a spade terminal connector splice in the pink wire, I checked continuity to ground again. It's now open.
I turned the key "ON" and connected the pink wire going to the fuel sender to ground and the fuel gauge started rising slowly (the voltage stabilizer is back in the circuit). I started the engine and everything seems normal. The fuel gauge stabilizes about 5/8 which is about right.
Where did my short go?