Blackstone
Brigadier
As you clearly pointed out, war is only one of the multiple ways US could employ to check China's reemergence to its traditional residence atop NE/E/SE Asia. War for the US, in fact is the worst option to maintain Asia primacy for many reasons, chief among which is the kind of war required to 'put China back in its box' could lead to total war that would be long and might escalate to usage of nuclear weapons by both sides. While China has no appetite for both long conflict and nukes, US has even less appetite for them. Far less. What it means is other than the Pentagon and a few neocon/neolib (liberal imperialists) think tanks and politicians, there is little or no argument for goading China into a war that US has no assurance it could "win." whatever winning means in this case.The two scenarios you presented are just two different points on the same spectrum, the spectrum of US power in East Asia. The relative decline of the US is more about China's rise than America's fall, war is one way to check China's rise. The sooner they do it the closer they can get to maintaining primacy, by 2020 they can only hope for co-dominion, and I expect that by 2030 they'll be on a path toward relinquishing hegemonic intent altogether in East Asia.
Time is actually on the side of US peacefully accommodating China, because the power gap between the two great powers are narrowing, and shows no sign of flatlining. US, even with support of Japan and Australia, can't be assured of "victory" today, let alone five or ten years from now. After that, it really doesn't matter how hard neocons/neolibs push for war, because the warhawks would be steamrolled by John Q. Public.Give it some time. The US fought plenty of wars against Mexico and Canada in the beginning. It'll take generations of being hopelessly overshadowed by a greater power for the populace to give up resisting altogether. Vietnamese resistance is already much milder now compared to the 70's and 80's, and it's only since the 90's after the fall of the USSR and the rise of China that Vietnam's resistance has become hopeless.
Bottom line is should there be a unified Korea without war, time would probably make it more aligned with Chinese interests than less.