How can we reconcile Allah's saying: "And We would not punish until We had sent a messenger" with the Hadith: "Indeed, my father and your father are in the Fire"?

There is no contradiction between the verse and the Hadith. Allah's saying: (And We never punish until We have sent a Messenger) indicates that Allah does not punish anyone until proof is established against them by sending a messenger. For those whom the Messenger's message reached, the proof has been established against them. As for the Ahl al-Fatra (people of the interregnum) whom the message did not reach, they will be tested on the Day of Judgment.

The two Ahadith: "Indeed, my father and your father are in the Fire," and "I asked my Lord for permission to seek forgiveness for my mother, but He did not grant me permission," indicate that the parents of the Prophet, peace and blessings be upon him, are in the Fire. This is because the Sharia of Ibrahim (Abraham), peace be upon him, reached them, and they were not among the Ahl al-Fatra. Scholars have used this as an argument that the Prophet's father was among those whom the call reached and against whom the proof was established.