How do we reconcile Allah's saying, addressing the Prophet (peace be upon him): "Should you associate [others with Him], your work will surely become worthless," with the fact that the Prophet (peace be upon him) was forgiven for all his past and future sins?

There is no contradiction between the two verses. The verse from Surat Al-Fath (Indeed, We have granted you, [O Muhammad], a clear conquest, that Allah may forgive you your faults of the past and those to come) is a firm truth regarding the forgiveness of the Prophet Muhammad's sins. As for the verse from Surat Az-Zumar (If you associate others with Allah, your work will surely become worthless), it hypothesizes polytheism and does not state that it occurred from him (peace be upon him). Its purpose is to alert the Ummah and warn them against polytheism, because prophets are infallible from it.