"The wire oxydized because it was in contact with OXYGEN, not heat"

You're setting up a straw man. Nobody claimed that oxygen wasn't involved in the transformation of copper metal to copper oxide. Any chemical reaction, such as the oxidation-reduction reaction that copper wire experiences when it becomes copper oxide and begins to conduct poorly, proceeds much more readily with increasing temperature. Indeed, in this case, the conversion of copper metal to copper oxide is caused by overheating. Anything you can do to prevent overheating will forestall the conversion of the copper metal to copper oxide

"...and the failure mode is shorting between the adjaecent [sic] un-insulated wire sections.

This statement just begs the question of what caused the insulation to break down in the first place. Heat causes it. The heat has two sources: convection from the hot engine, and voltage drop in the wire. You can prevent convection from the engine by wrapping the harness in fibreglass heat-insulating tape, and you can prevent heating due to voltage drop in the wire by using a larger-gauge wire.

"Excessive heat causes copper to become brittle, usually characterized by a brown appearance and how it breaks easily without its normal flexibility."

Another straw man. That's exactly what I said, but you're trying to make it appear that I said something different. That brittle, brown stuff is copper oxide.

"Boat cable is pre-tinned, easier to work with, less prone to corrosion where exposed, and more flexible which all makes for a better harness and easier for the OP to work with. The normal household THHN etc. will not be flexible enough for automotive use, and the stuff you buy at autozone et al can be of dubious quality."

I never said you shouldn't use good quality wire; just that if you're going to wrap it in fibreglass heat insulating tape, you don't need to use mil-spec or boat wire because it is not going to be exposed to any heat in the first place. For example, I wrapped the headers on my hot rod with fibreglass heat-insulating tape. When the engine was running full-tilt on the dyno and headers were red hot, the heat-insulating tape was only warm to the touch.

"Mercedes didn't use "the smallest gauge wire they could get away with", they used the correct wire gauge"

You're distorting what I said. Yes, they used the correct wire gauge if everything works as it did when it left the factory. But as the factory-stock wire degrades, and when the connectors get dirty, there's no conductivity headroom. That's what I meant by "the smallest gauge wire they could get away with". By using larger gauge wire, you have much more conductivity headroom to prevent excessive voltage drops.

"I'd like to see your voltage-drop calculations on the wiring"

Sure, no problem. The current-carrying capacity of a wire is proportional to its cross-sectional area, which is in turn proportional to its diameter squared. Each AWG gauge has a diameter approximately 1.12 times that of the next smaller gauge. That means that, for example, AWG 16 gauge wire has a diameter 1.25 times as large as AWG 18 gauge wire (because it's two gauges larger). Therefore, AWG 16 gauge wire has a current-carrying capacity of almost 1.6 times (1.25 squared) that of AWG 18 gauge wire before excessive voltage drop is encountered. That's a lot more headroom!

Methinks thou dost protest too much.