“But what Paul states here is a general axiom that would be true in any number of contexts that go well beyond personal redemption, as has been demonstrated many times over elsewhere in Scripture. As just one of myriad examples, the temporal authorities attempted to keep Peter in prison. God wanted him out, so he got out. God always gets his way.”
Nowhere in Scripture is Romans 8:31 used “as a general axiom that would be true in any number of contexts”. You are using it in that sense, not Scripture. Paul and the other writers were quite capable of quoting Scripture when they wished to use a verse, verses, or a passage of Scripture to make a point.
Truth is truth. You can put a verse label on it or you can put it in Paul’s mouth or King Nebuchadnezzar’s mouth and its still true. It’s just true that if God sets his favor on someone, it does make that person undefeatable. Do you deny, for example, that if God wants to set Israel free from Egypt, that the Pharaoh has any chance at all of stopping him? Or do you think King Nebuchadnezzar is wrong to say that God does whatever he wants in heaven and earth, and no one can stop him or even challenge what he is doing?
Paul is saying in Romans 8:31 what Hebrew Christians already knew from these and many other stories of God setting his favor on nations and individuals and overcoming their enemies for them against seemingly impossible odds. He is just applying that general truth to the specific instance of individual salvation. The Sovereignty of God is taught in one form or another on every page of Scripture.
And it occurs in our lives too, in so many different ways. The most important of these is that God will not fail to bring his sheep home to Heaven, no matter what obstacles they may face in this life, whether material or spiritual. That is the message of Paul in Romans 8:31.
But God also rules over the affairs of humankind. He raises up kings and takes them down. He sets his favor on a young sheepherder named David, and though David must quit his home and hide from King Saul in the caves like a common criminal for years, God does not forget David, but in due time puts him on the throne, just as he promised.
If God is for David, who can be against him?
If God is against Saul, who can help him?
If you believe the above two propositions are true, regardless of who said them, why would you have a problem applying that truth to someone living now? God is the same yesterday, today, and forever, right?