I like your creative thinking.
But starting a 200 HP engine to run its tiny little air brake compressor does not sound efficient to me. You don't get 200 HP worth of air charging. You only get what that little compressor can do -- maybe one HP worth -- wild guess.
Powering a small compressor OR generator with a small engine is more efficient, because the small engine is operating closer to its maximum efficiency.
And I'm not so sure it is a good idea to start an engine every day for just a few minutes. Starting a cold engine is very hard on it.
I do like the concept of "straight from compressor to water pipe". Sound thinking.
But your last item on this post, about an air motor to power a generator, will not work so well. As was mentioned in an earlier post, air motors consume large air volumes. My die grinder draws down my shop compressor tank faster than any other tool I have.