How can we reconcile the Quran's not being eternal in the sense of God's eternality, with the statements of the Imams who describe it as primordial (azali), eternal (qadim), and uncreated (ghayr muhdath), and their excommunication of whoever claims it is created (hadith)?

It has not been transmitted from the Salaf that the Qur’an is ancient in its essence. Rather, they agreed that it is the uncreated speech of Allah, and that what is created, according to them, are the entities and the attributes subsisting in them. Whatever in their statements might suggest this meaning, what is intended is the attribute of speech, not the Qur’an specifically. The ambiguity is removed by distinguishing between the type of speech and its individual instances; for the speech of Allah is ancient in type but originated in its individual instances. This issue becomes clearer by distinguishing between essential attributes (Sifat Dhatiyyah), active attributes (Sifat Fi'liyyah), and essential-active attributes (Sifat Dhatiyyah Fi'liyyah) like the attribute of speech. The doctrines of people regarding the issue of speech are many and complex, and none is sound except the methodology of the Salaf.