Very interesting theories...
I actually bought a generator once at a discount. It had been sitting for several years, brand new, test run at the factory, then nothing more.
It was crated up for shipment, then the company went out of business, and years later I bought it in that exact condition at a salvage sale, still in the crate.
I looked the engine over and thought well...
Changed the oil, Put some diesel fuel to it, and hit the starter. It didn't start.
What to do...
Took out the injectors, soaked them in a tub of diesel overnight, put them back in the engine and tried it again. Viola, it ran perfect!
That generator is now one of our test generators for my business. We use it for backup, and we also use it for testing out new control systems, and stuff like that. The engine has thousands of hours on it now and runs like a top.
If it were me, with the boat, I'd do just that again. I like the idea of getting some oil moving around by the starter prior to starting. I like the idea of a new impeller in the raw water system, but any thing else is not really going to get you anywhere I believe.
Remember diesel fuel is a oil. It is a lubricant.
I think that the engines will be the least of the issues. There will be other things that have problems, but they are not all that big of a deal for a boater. Things like pumps and the like.