If you had the incoming US president's ear for a half hour, what would you want to make sure he understands?

I think a better question is "Is there any possible combination of words that could change his behavior?" It doesn't matter what "understanding" you could impart to him, because he doesn't care. It would be like water off of a duck's back. You'd have to make him "understand" something like "Here's a bribe to not be a shitstain" or something else that anybody here almost certainly wouldn't and/or couldn't do.

That I just spit a gigantic mucusy lougie into his ear so deep that the fucker will be pulling chunks out for the next decade.

Fuck, if I had this opportunity I might even be tempted to take up smoking first.

That he is going to die a miserable death