While this isn't a complete answer to your question, there are two points in your description that deserve noting.

First, the UN resolution came right when President Nixon was pursuing a warming of relations with the People's Republic of China. He had already announced his intention to visit the PRC several months before, and was to make his very famous tour of the country a few months after the resolution. This shift in American priorities and policy was a major factor in the UN resolution.

Second, while the US government has stated in various forms that its interests are for a peaceful resolution of the Taiwan "issue", even providing military support to the ROC, the ROC is technically not a US ally. This is a result of there being no formal diplomatic relations between the two countries as the US does not recognize Taiwan or the Republic of China as a nation, and formally does not support Taiwanese independence (of course, neither does the ROC currently). In reality, this is of course mostly a formality dictating language rather than anything else-- for example, instead of an embassy, the US maintains the American Institute in Taiwan (AIT), a non-profit private corporation that by sheer coincidence is heavily involved with the US State Department, while Taiwan has the Taiwan Economic and Cultural Representative Office in the US (TECRO), which is a similar institution.

Ask and you shall receive