There is no silver bullet, but that situation cannot get much worse than it already is. Even if the country collapsed into anarchy, the chaos would at least provide the opportunity for some to escape.

Organized evil is more harmful than any sort of chaos. The only exceptions involve WMD deadman switches.

I doubt it would collapse or even undergo anything much of note. I imagine if Kim Jong Un was assassinated, he would "retire" and be replaced with someone else. The government is much more than just the one guy.

Not suddenly, but maybe after the 4th or 5th replacement got assassinated in short order. Keep killing them as long as they remain hostage takers who don't surrender.

There are easier and better ways to start a war with North Korea.