Hello and welcome
Concerning Japan, that might interest you:
http://www.mod.go.jp/e/d_policy/index.html
I agree with what you are saying here and I just want to add one thing. Indeed, I would consider that Japan will be nuked to some extend (US bases and major cities) and I'm convinced that part of the population will die from hunger. However, something else might add to the bad situation there: natural disasters. I have no real knowledge about that but I would expect that the level of nuclear exchange results in an increase of earthquakes, volcano activites and tsunamis.
About its involvement, Japan will probably not send ground troops outside of its borders but the air force might be engaged as far as South Korea and the navy should be involved in accordance to the treaty with US.
The article V of the treaty with US will allow Japan to act, or so I think, as UN has collapsed. Nevertheless, the article 9 of their constitution will prevent them to go too far. Finally, the government interpretation of it and the extend of popular support will modify this as well. For my part, I even consider that the JGSDF is even involved in South Korea as the collapse of South Korea has come to be seen as a direct threat to the very existence of Japan.
Here are what I think to be the important points stated by the government of Japan. The minimum level necessary for self-defense will change greatly under the Twilight War.
B. Requisites for Exercise of Right of Self-Defense.
The use of armed force for the exercise of the right of self-defense under Article 9 of the Constitution is confined to corresponding to the following three requisites:
(i) there is an imminent and illegitimate act of aggression against Japan;
(ii) there is no appropriate means to repel this aggression other than the use of the right of self-defense; and
(iii) the use of armed strength is confined to the minimum level necessary for repelling.
C. Geographical Scope of Exercise of Right of Self-Defense.
The geographical scope of use of the minimum force necessary to defend Japan as the use of self-defense right is not necessarily confined to the Japanese territorial land, sea and airspace. Generally speaking, however, there is no specific definition of how far this geographic area stretches, since it would vary with each individual situation.
It is, however, not permissible constitutionally to dispatch armed troops to foreign territorial land, sea and airspace for the purpose of using military power, as a so-called overseas deployment of troops, since it generally exceeds the minimum level necessary for self-defense.
By the way, if any one knows Japanese, I'll be interested in having a full translation of that website (Ground Forces, as always, have been lazy). Santa is late for me.
