Is the president of the US legally entitled to arrest foreign nationals? China and S Korea seem to have put in all the work to make this meeting happen, so they might have something to say. Besides, I doubt that he would be bothered with any action that didn't make him money.

What if the person at the talks is a body double?

Not going to happen. Arrest him for harming North Koreans? Opportunistic POS trump would never do that.

North Korea could launch missiles at the U.S. The U.S. would become the focus of international condemnation, since we would have no authority to arrest a world leader in another country. We are not entitled to make unilateral decisions of this type.