How did verses descend upon the Messenger, may God bless him and grant him peace, in response to human events, while the Qur'an was written in the Preserved Tablet from eternity?

If the question is about the reason for the revelation of verses that coincide with events, despite the Quran being inscribed in the Preserved Tablet, then the answer is that Allah Almighty is All-Knowing of everything. He knew from eternity what would happen and that the Quran would be revealed in it, so He inscribed it in the Preserved Tablet.

And if the question is about the Quran not containing mention of all unseen matters, then it did include news of unseen events that occurred just as it foretold, which indicates that it is from Allah. Examples of this include: the defeat of the hosts, the victory of the Byzantines, and the establishment of the believers in the land. It is not necessary for Allah to inform of all unseen matters.