I've used ChrisFix's "Super Clean" series a couple times to very good effect, and recommend it to you. Don't know if it will answer all questions you presented here, but it's very thorough.

Okay so I would just drive it down to the local do it yourself car wash pop the hood and go to town. I have done this many times with both gas and diesel engines. The only problem was once getting water in the starter and it not starting till the starter dried out. Just don't get too much water on the exhaust manifold if it's hot and stay away from the fuse box.

I know it sounds scary but really the whole thing is designed for rain and splash. It will save you a world of headache both in time and clean up from the mess made by cleaning.

It might not get you to 100% but it will do most of it for you.

Shopvac 1st, with a bag, to get all the infections stuff reduced.

That is the single most-important point.

Use a strong, not anemic, one, with a small nozzle, to concentrate the suction.

The safest method, after that, would be damp rags ( washing-soda or baking-soda solution to make the water a bit more alkaline & low-surface-tension ), perhaps on a reacher.

I learned of the damp-rags method from electricians, who were getting the dust/junk out from a building's transformers ( the power to the entire building had been killed, for the operation, which is required every few years ).

That way you have no risk of getting water/cleaning-solution into places where it'd do harm ( like electrical connectors, e.g. which can then kill some of your vehicle's systems )

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