The happy end would be a unification of north and south.
Unlikely. Neither the Chinese nor the Russians would be extremely happy with a close ally of the States on their doorstep instead of the current heavily fortified buffer zone.
The Chinese would be much more concerned than the Russians. The Russo-Korean border is only a few kilometers long, and I don't think that the NATO will install bases anyway near that narrow strip of land. On the other hand, the Sino-Korean border runs through three strategically important provinces of China.
I do not agree. During the first war in North Korea could trigger a mass Exodus of civilians and cause of the migrant crisis. In the second, the first target of an American attack are silos of nuclear missiles. The radioactive cloud could carry out the wind on the Russian territory.