aoe3rules, obviously there is incentive to not abandon UK, but no more than any other major trade partner with the UK. I would argue that the Commonwealth countries would not join the war simply because they are Commonwealth but to secure their own interests. This is contrast to the past when these countries viewed themselves as kin to the UK, in which the obligation to fight existed regardless it was in their best interests.
Why would North Korea and China invade South Korea? Just because they tried it before? With South Korea's sunshine policy and their mutual desire for re-unification I don't see why they would have to resort to violence, simply because the rest of the world was at the onset of a third world war. North Korea's official policy is to seek reunification without external pressures. Getting involved in a world war is the worst way to avoid international involvement. If Koreans want anything it's their independence. China is officially for the peaceful reunification of Korea, despite that such a reunification might be against their interests. They would absolutely not profit from a military takeover by North Korea. South Korea is a major economic power in Asia now and China very much values it's investors.
Actually, I would say that the Chinese people are more for a war with America than their government is. There is a lot of anti-American sentiment, and a strong held belief that the Western world is trying to interfere and exploit China. However, China's government would never go to war with America. It's vast treasury is almost all American dollars. If the two went to war they would both be economically crippled. It would undo decades of progress, if not more. There's just simply nothing to gain from it. Countries don't fight for nothing...
Nigeria and Indonesia are hardly imperialist countries bent on exploiting their neighbours... I don't we have to worry about them starting the next world war...
I would hope that USA can hold China farther than 20km from the coast, because otherwise they'll be chugging missiles. USA has significantly superior military than China, except perhaps infantry-wise thanks to sheer numbers, but it can not expect to tackle a giant like China without incurring significant losses, including civilian deaths. They're are many ways of hitting America that do not involve swimming. There are enough die-hard Chinese in America already. =D
It looks more like no war at all.