How sound is the attribution of the Byzantines (Romans) to Isaac, peace be upon him? And is the Hadith: "The Hour will not be established until seventy thousand from the Children of Isaac invade it..." (etc.) an authentic Hadith, or is it not ثابت (proven/established)?

The Romans are descended from Esau, son of Isaac, son of Abraham. The hadith is authentic, stating that seventy thousand of them will conquer a great city like Constantinople, not with weapons but with takbir until it falls. They will enter it to take spoils, then they will hear about the emergence of the Dajjal and return. Some have found it problematic that the conquest would be at the hands of the descendants of Isaac, who are the Romans, thinking that the conquerors should be Arabs (descendants of Ishmael). However, it is established in the foundational texts and the words of scholars that the Romans will embrace Islam in the End Times, and some of them will be part of the Muslim army. It is possible that "descendants of Isaac" in the hadith includes Muslims among the Romans or others from Isaac's lineage.