How is it possible to reconcile the Hadith, "If the Hour comes while one of you has a sapling in his hand, let him plant it," with the Hadiths that indicate that the Hour will only come upon the most wicked of people, or that it will not come upon a believer?

There is no contradiction between the sacred texts. To understand the seemingly contradictory hadiths about the Hour and good deeds, one can consider one of the following two interpretations: First: The intent behind the Hour's advent is the appearance of its major signs. In this case, the hadith (definition) is an encouragement to work and cultivate the earth even as the Hour approaches. Second: The hadith is a parable, meaning: do not despair of working even if you do not see its fruits yourself, for others may benefit from it.