Personal I feel both Koreas will sit out this conflict pending something seriously unexpected.
I would agree, but with the following proviso.
If Japan joins in a war, then there will be a lot of secure rear-area bases in Eastern Japan because they are distant from the Chinese East Coast
However, Chinese Manchuria is close enough to all of Japan to launch significant numbers of missiles and aircraft.
The issue is that there is a thin strip of North Korean or Russian land to cross over.
Given that North Korea is dependent on China for 90% of its trade and for overall security, North Korea can't afford to see China lose a war.
Therefore I don't see North Korea protesting very loudly if Chinese missiles or aircraft cross over their territory.
And what is the US going to do anyway?
Start a war with North Korea, which will turn into a land war between the Chinese Army and the US Army on the Korean Peninsula? Remember that the Korean Peninsula is an extension of the mainland China landmass.
Plus North Korea is under maximum economic sanctions in any case.
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Alternatively, I don't see the Russians complaining very much either if Chinese aircraft cross over a small strip of Russian territory. After all, these Chinese aircraft are on their way to attack US airbases in Japan.
Again, Russia is already under maximum sanctions in any case.