"Not to mention China would never let the US actually directly engage Russia militarily."
Are you sure? If Obama started threatening NATO action to get Putin to reverse the Crimea annexation, do you actually think China would threaten military action against the US to get us to stop? No, they'd probably just say they "appeal for calm" and ask Putin to return to diplomacy behind closed doors.
Again, all these people, Xi Jinping, Obama, Putin, aren't insane. Just as in the Cold War, everyone understands that a war between any of them would basically be the end of the world. But they also all understand that the West would win the war, if there was one. As long as this is true, the US can and should use the threat of military force to see to it that international law is respected.
It's too damn bad, really, because even if this all ended tonight, the Crimea crisis has set us back several years in terms of integrating countries like China and Russia into the world system so that they don't have to be adversaries anymore.
Indeed, there is a remote chance that if the West plays it right, this crisis could alienate China from Russia toward the West. Very unlikely, but possible. China might actually finally become that "responsible stakeholder" we have been desperately hoping for ever since it became clear that China was or would become a superpower.