How can we reconcile Al-Miqdad's statement on the day of Badr: "So go forth, you and your Lord, and fight; indeed, we are sitting right here," with the fact that this verse is from Surat Al-Ma'idah, which was revealed after Surat Al-Fath?

The aforementioned Hadith is authentic, and the verse referred to is from Surat Al-Ma'idah, which is unanimously considered Medinan. Surat Al-Ma'idah was revealed in parts (piecemeal) and not all at once. The Prophet (peace be upon him) would specify the place of the verse when it was revealed. The claim that the entirety of Surat Al-Ma'idah was revealed during the Farewell Pilgrimage is incorrect, as there are verses within it that were revealed much earlier, including the verse that followed the incident of Badr.