If the US didn't honour its commitments and let Taiwan fall, Japan, Korea and all other neighbours would no longer be able to rely on US protection and seek their own weapons of mass destruction. The instability of that scenario would almost certainly lead to WW3 and be more detrimental to US interests than a skirmish over Taiwan.

> Supporting pro-China elements in Taiwan, spending a generation 'educating' (i.e. propaganda) Taiwanese that they are in fact part of 'mainland China', using coercive economic incentives ... I think that Taiwan may ultimately join China through a slightly shady but nevertheless 'legal' popular referendum.

You are ignorant. Taiwanese identification with China has been falling steadily for decades. According to current polling, somewhat less than half would support a declaration of independence and the inevitable conflict that would come with it, but a small single digit percentage are willing to join the PRC.