Most model engines are a magnesium-aluminum alloy. My understanding is that the magnesium reduces the reactivity of the aluminum by creating a magnesium-aluminum oxide coating over the surface. Methanol dissolves the normal AL2O3 coat that protects the metal. In the case of a mag alloy the oxide layer is around 3x the thickness of that found in typical aluminum. Just to make it all worse, the alloy found in the typical outboard was a zinc-aluminum alloy. A metal even more reactive in the presents of Methanol than pure Al.
Model engines also run an oil-fuel ratio that is impossible to achieve as a practical application in a large engine. Further, castor oil does buffer the corrosive effects of the alcohol. They end a flight dripping with goop. Can you imagine running your boat with a 15:1 mix of castor and klotz? Sure to impress the DNR with that self-propelled stink-bomb.
In the case of methanol-rated carbs, they are hard anodized over a more corrosion-resistant alloy. They are still slowly damaged, but it becomes a matter of many years, not a single season. They are sold as "methanol resistant". Everyone I know has fitted a quick-drain to the carb to dump the bowl. Methanol is hard on the rubber, requiring a synthetic for diaphragms, gaskets. (methanol + holly stock power valve= methanol-waterfall into the manifold,)
As far as Methanol injection goes, the idea was to cool the incoming air charge through evaporation, and reduce it's non-compressable density by absorbing the free moisture in the air. It acted a bit like a chemical inter-cooler. Setup properly a mix of 55% water:45% Methanol injected with 91 octane will run in a 13:1 engine just fine. Methanol injection is making a return in the high performance diesel engines. (if there is such a thing) My attempt to use it was, well, lets just say it was slightly less than a raging success-unless the measure of success was just how far I could make pieces of an hirth engine fly. (I used a Snow's kit to build my grenade, er, I mean drag sled, resulting self-disassembling engine was my fault, not the kit's)
Having Being a Small Engine Mechanic for over 30 yrs ....You are quite correct!!! One more thing on ethanol and 2stroke motors is as soon as you add oil to the mix the fuel (be it ethanol or normal unleaded fuel) will start to deteriate, so that after aprox 2-4 weeks of standing in a unused fuel container it has probably gone past its use by date....You only have to smell it to know...If its real old you will get an instant headache...I have pulled numerous carbies apart only to find a yellow gunk inside that blocks everything and is nearly impossible to remove...so if you are going to use this fuel in older engines and not use them for a while ...drain them and fill them with a mix of diesel and kero...for storage