I have an Israeli passport with a ten-year visa to the US and a multiple entry visa to China.
Is it possible for me to take a ferry from Xiamen to Kinmen and then to Taipei and come back to Xiamen?
The answer on your question: YES
As written on Taiwan's goverment website you may enter to Taiwan for up to 90 days without visa.
With multiple entry visa to China you may return to Xiamen as well.