>why is it so difficult to meet them once (somewhere outside of North Korea and China)? The cost is negligible compared to a large salary.

It wouldn't matter. They'd hire some actor to do it. If you insist that they take precautions to be sure the person in the video interview looks like the guy they meet, they'll do that too... but the one doing the work will do so remotely from Pyongyang. There might be technology fixes for this, but they almost certainly involve isolating the United States' internet from most of the rest of the world.

Yes, but it makes it a bit harder. Every verification step lowers the risk, if only a little bit. It does matter that much.