I think I have already answered to your "American calculation" - rational or otherwise. That is, regardless what American calculation is. Or to be more specific, whether the US will intervene or not, or whether it will go nuclear or not. It might affect the tactics somewhat, but it won't change what China will do ultimately. China's position on Taiwan has always been clear, consistent and unambiguous.
The point being, China will always be willing do more for Taiwan, up to risking MAD. Whether the US will ultimately be willing to do so for Taiwan in order to maintain its "primacy," the US will have to decide. Its decision won't affect China's, which has been pretty clear and consistent from its public statements, laws, constitution and past behaviors.