Japan could very well join the fray, or at least allow us to operate from there, in the event of PRC aggression against the ROC.
Korea would be an interesting situation and I am almost willing to bet that the North would take advantage of such a situation to try and go south while we were distracted. In fact, a more likely scenario IMHO, would be the N. Koreans going south first, getting us involved over that...and then China making its play for Taiwan.
In either event there is Guam for long rang missions, tanker assisted fighter cover, or long range B1 or B2 missions.
My point is that the PLA is not led by idiots like the Iraqis. They would not let us set up on Taiwan - our bases would be attacked instantly by missiles and bombers, long before they became operational.
It keeps coming back to political will - If it were up to me, my FIRST strike would be against the Three Gorges Dam, with a nuke if necessary. But I'm pretty bloodthirsty. Need to ask yourself "What would Hillary do?" since, given Republican idiocy, she's likely to be President. I doubt there is the political will to do what is necessary.
Given the political will, we could win a naval war and stop an invasion. But that is a pretty big if...
If the North goes South in Korea, given the current state of the North Korea, it is going to be VERY roughly handled by ROK forces. The commies don't train enough on their equipment, don't have enough to eat, enough fuel, etc. Again, it would come down to political will. A appeasement minded South Korean government might hamstring ROK forces.