The Scriptures indicate that there are several important doctrines in play simultaneously. That this is so should not surprise us...we are dealing with a God whose mind is beyond infinite. It is transcendent. We are trying to keep up from language given to us describing how a transcendent and sovereign God thinks and acts.
To discuss God's extension of mercy upon the world, we will need to deal with foreknowledge and predestination at the same time. So, if you are defining "mercy" as God extending the ability for a man to make a completely self-determined decision to trust God, or reject Him (again, completely unaffected by God's control over that decision), then God has to have no idea who will be saved and who will perish. If He knew, then the decision would not be taking place at the moment of "decision", but sometime prior.
If a man is really "free", then God really does not know what he will choose rendering God's foreknowledge deficient. I think we all agree that foreknowledge implies knowing beforehand (in time/space history) the outcome of a particular event. But, for "freedom" to exist, God cannot have prior knowledge of the outcome. This is what gave rise to the "Open Theology" movement, a cult of monumental proportions.
We, OTOH, maintain that the Scripture makes it abundantly clear that God knows every detail that will happen in the future. This is one more reason He can provide prophetic statements about future events with certainty. His foreknowledge is actually the understanding He possesses of the decisions He has already made and is executing after the counsel of His will.
Therefore, taken together, Rom. 11:25ff is simply saying that the day is coming when, irrespective of Israel's stubborn heart, incalcitrant behavior, firm resistance to Yahweh, He will break their hearts, change their status from "enemies" to elect and rescue perhaps an entire generation. God knows the time when the "fulness of the Gentiles" will be complete and at that moment will switch over His outpouring of "mercy" and turn to the Jews, again. He is powerful to manipulate their will, exactly as He has done to the Gentiles. cf to Acts 13:48, "And when the Gentiles heard this, they began rejoicing and glorifying the word of the Lord; and as many as had been appointed to eternal life believed." Notice, not, "as many as believed were appointed to eternal life." but, "as many as were appointed to eternal life believed."
Taken out of context of Paul's argument, it is tempting to make the "all" of Rom. 11:32 imply "every human being". But, in context he is saying, the way God metes out mercy applies to every "nationality", both Jews and Gentiles (that is the local argument). When He wishes to soften the hearts of one group, he softens them. When He wishes to harden, He hardens and all peoples are subject to His way.
Were this not the case, Pharaoh would have been dealt a singularly raw deal being the only human that reached a point to be "hardened". This would be contrary to Rom 11:32. But, the text of Rom. 9 clearly claims Pharaoh is a picture, a type, of God making the clay pots he has set aside for destruction become unable to see Him, turn to Him, repent to Him. And the rebuttal question of, "How can He still find fault, for who resists His will?" is anticipated by Paul. Go read his answer. Tough luck, people. We have no business saying God cannot run His universe.
Rom. 11 is the same argument on Israel. They have been temporarily hardened and considered "enemies" for the sake of opening the way for the Gentiles to receive the mercy. And, incidentally, since there is really no such word as "Gentiles", it is simply "all other nations", this means that the people outside of Israel are currently getting the great dose of mercy now. But, that window will close someday and God will turn back to Israel to work on them.