The state of war still exists. Cease fires end all the time. I doubt the armistice says anything at all about the internet or 'cyber warfare' in general.
This is all also assuming it was the US government that did it.
This is true. That sort of thing may be inevitable though. May not happen for another generation or more but eventually things will have to change in North Korea. When they do those refugees will still be an issue for China.
Korea as a whole.
Nice try, South Korea is an occupied territory, it's quite a big occupying force as well. If they were not there unification would have taken place a long time ago. Poor Japan, occupied all these years and now total ruin.