I don't recall ever seeing your understanding of Romans 11:30-32. Can you tell me your interpretation of that passage and whether you see any relation to that passage and Romans 9:15-18 in the sense that it tells us who God wants to have mercy on?
You said, "How does it make sense for you to say that he withholds His mercy from others in light of what it says here:" and you brought up
Romans 11:30-32
"Just as you who were at one time disobedient to God have now received mercy as a result of their disobedience, so they too have now become disobedient in order that they too may now receive mercy as a result of God’s mercy to you. For God has bound everyone over to disobedience so that he may have mercy on them all." --I assume the reason you brought up this passage is because of the idea of God having mercy on all the disobedient.
Notice that Paul is referring here to the mercy of God toward Gentiles as a result of the disobedience of the Jews. His "all" here, has to do with the inclusiveness of all peoples, both Jew and Gentile. It is not saying that God will indeed have mercy on absolutely everyone henceforth. Are you a Universalist?
But also, there is a use of the passage in which Paul rhetorically includes absolutely everyone [since mercy is the only way out for any (assumed though not spoken)] --and is not saying that absolutely everyone will, or even possibly can.
(You might note that you must assume some provisional mercy by God upon absolutely everyone, in order to avoid universalism in saying mercy on all. You must say, "The term, "might", means it may or may not actually come to fruition. It provides for the possibility." I bring this up, in case you object to me saying something is assumed though not spoken, because that is what you must do too, unless you are a universalist.)
I think it is a combination of the two. Both apply. This is, of course, just my take --I don't represent Calvinism in this, though I have heard Reformed Theologians and teachers use one or the other ways of looking at it.
Also, I find it necessary to object to your phrase "withholds his grace", once again. It is not as though he unfairly keeps it from some. No! -- He unfairly gives it to some. We all get what we deserve, except those of us to whom he has chosen to show mercy.
Romans 9 says,
"15 For he says to Moses,
“I will have mercy on whom I have mercy,
and I will have compassion on whom I have compassion.”
16 It does not, therefore, depend on human desire or effort, but on God’s mercy. 17 For Scripture says to Pharaoh: “I raised you up for this very purpose, that I might display my power in you and that my name might be proclaimed in all the earth.” 18 Therefore God has mercy on whom he wants to have mercy, and he hardens whom he wants to harden."
I love this part of the Chapter, although I particularly delight in the whole chapter, as you might guess, since I'm of the Reformed persuasion, and indeed I love the whole Book of Romans.
Of course, I see Romans 9 as related to Romans 11, (and the whole rest of the book, for that matter). Earlier in Romans 9 in the discourse of the potter and his clay vessels, some chosen for honorable use and some for dishonorable use, the point is clearly made that this is all the work of God, and that he will do as he sees fit with what is his by right of ownership (i.e. by right of creation and ultimate lordship as God). The clay cannot speak back to the potter, nor does man have the status to judge God.
When the potter makes a vessel for a specific use, he will do to it as he pleases, and will not consult the clay as to its preferences or wishes. (My note: "It is, after all, not even alive.") So he will have mercy on whom he chooses to have mercy, and harden whom he chooses to harden.
You, I assume, like many others here, want Romans 11 to say that God has mercy on everyone, (having died for everyone), but that through Free Will they have rejected him. I do not deny that absolutely all indeed have chosen to reject him, but Romans 9 shows that "it does not depend on human desire or effort, but on God's mercy" who will receive Salvation. This also does not deny, as I have repeatedly stated on this forum, that man indeed does choose God, but God's mercy does NOT depend on what man desires or chooses, no matter the degree of his integrity of choice.