The argument about a post-judgment repentance is partly one of historical plausibility. It was clear to Paul that the inclusion of Gentiles was not provoking sufficient jealousy for Israel to repent as a nation. Paul expected Israel to suffer catastrophic judgment. 4 Ezra and 2 Baruch show that the Jews could interpret AD 70 as judgment and express repentance after the event. So it seems historically plausible to think that Paul imagined that his people would repent after the catastrophe. Historically speaking, there is no reason to think that he would have expected the Jews as a people to be utterly wiped out by the war.
But I think that the quotation of Isaiah 59:20 points to this sequence: God comes to punish Israel, Israel repents after punishment, Israel is restored.