If China did “come over the border”, would they use tactical nukes?
They don’t need nukes. All of North Korea’s assets are pointing south. If NK moves them north, China will see that as a threat to them. If China comes over the border, it will be a short fight.
Kim’s in a bind. The bigger question is what Kim will do with his nukes. Maybe that is why the ships are there, protecting Japan with the SM-3’s which can shoot down ballistic missiles.