In any hypotheticals that I propose between Taiwan and China, generally I don't factor US involvement because it's too much of a swing factor.
I don't think so. American involvement is a definate. No sane American president would let the Chinese get away with that. Even if the Chinese only attacked the outlying islands, the Americans would send a couple of CVBGs as a warning and demand in the UN that they withdraw. The Americans would do this because if the Chinese continue onto "mainland" Taiwan they would have to get involved because of their treaty obligations so they would want to stop any war before it starts. Peace is in everybody's interest in the Strait.