Regarding Korea, even if things escalate, they can stay limited and then De-escalate. Maybe it would be beneficial in the long run, if the two parties give themselves a black eye in a temporary scuffle. It might highlight the unseemly cost of a total war for both sides and bring them back to the negotiating table.
As for China, this $1 trillion project is going to address that momentary vulnerability, access to the Pacific is not really relevant beyond posing as a layered defense and it being a direction of attack by the US-China need only build a deterrent in that direction, it isn't focused in increasing its trade with the Americas:
Note that every major port along the sea based route is going to be accompanied by a land based corridor. Thus every potential choke point is either by-passable or bolstered by land. This would make the sea borne trade flexible, and if need be, completely redundant. India is being bypassed because it chose not to participate in the transit scheme. However trade between India and China will continue to grow and its own major rail and road trunk networks will be connected to surrounding corridors.
China needs unrestricted access to Afro-Eurasia to meet all its needs. This is doable within ~20-30 years. The Pacific is going to be of increasingly little relevance, there is nothing there and the US will comprise a smaller and smaller percentage of Chinese trade over time as Africa, middle east and the rest of Asia catch up to the rest of the world in terms of urbanization and industrialization.