You missed the point of drills and training exercises. They aren't actually invading DPRK, Just practicing. Amphibious landings, para drops, red v. blue battles, ACM, etc. Just normal stuff. They aren't forming battle lines and charging the DMZ. Of course the officers may have walk-throughs up near the line, but why not?

If DPRK can talk Mexico or Canada into joint drills to practice a US invasion, more power to them. I don't think they'll find any takers, though.

You need to understand that allowing other countries to train jointly reduces tensions (oddly enough, not with DPRK because they don't have any friends left). China routinely sends people to South America for jungle warfare training and has in the past run joint amphibious exercises with Columbia. Part of being friendly involves being friendly. Talking to each other is how militaries avoid wars over stupid stuff caused by misunderstandings. That one of the ways NATO and WarPac avoiding killing each other during the cold war period. Each had observers in Germany.

Of course, you can let the politicians work things out, but be ready for a war every couple of years because they're all busy measuring their dicks and arguing and could talk their way out of a paper sack.