We can go back and forth about this ad nauseum. I'm not going to quibble with you over every trifle. Paul identifies those who were blinded (gk: hardened) as the non-elect. That pretty much settles it. It doesn't matter whether one believes God is the one who chooses, or that the choice belongs to the elect, we're told that God foreknows the elect. When one speaks of the elect, he is speaking of the elect of all time. They are not the ones who were hardened.
Neither does the use of the plural pronoun "they" when speaking of elect Israel's stumbling as a whole provide any difficulty. We often, and well within normal and natural usage, interchange plural and singular pronouns when speaking of groups. One often hears of the government referred to as "they" or "it." Where "they" or "them" is used when speaking of the stumbling of Israel, one can just as naturally say "he" or "him" without changing the meaning.
So, where you think "Have they stumbled that they should fall," must mean a plurality of individuals, it can just as easily be said, "Has he stumbled that he should fall," because "they" is referring to the collective.
Once you admit the Scriptural definition and use of the word elect, the meaning in vs 23-32 is clear, for no one who is not elect is foreknown, and no one who is not foreknown is predestined to be conformed to the image of God's Son.
The hardening has happened in part to the collective, and it's only temporary in regard to the collective. It certainly wasn't temporary in the cases of Herod or Caiaphas, or Judas. And Paul, being of the elect, was not one chosen to be blinded, but who but God would have known until his conversion? So, don't boast against the branches. There is nothing in you that makes you any more desireable than a natural one.