“This decree is as immutable and eternal as His holiness.” This certainly does not speak very well of the holiness of God!
We read in the Bible that God does change His mind and reverse His decrees when the circumstances make it appropriate for Him to do so.
Genesis 6:6. And the Lord was sorry that he had made humankind on the earth, and it grieved him to his heart.
Exodus 32:14. And the Lord changed his mind about the disaster that he planned to bring on his people.
Jonah 3:10. When God saw what they did, how they turned from their evil ways, God changed his mind about the calamity that he had said he would bring upon them; and he did not do it.
Jeremiah 18:7. At one moment I may declare concerning a nation or a kingdom, that I will pluck up and break down and destroy it,
8. but if that nation, concerning which I have spoken, turns from its evil, I will change my mind about the disaster that I intended to bring on it.
9. And at another moment I may declare concerning a nation or a kingdom that I will build and plant it,
10. but if it does evil in my sight, not listening to my voice, then I will change my mind about the good that I had intended to do to it.
Jeremiah 26:3. It may be that they will listen, all of them, and will turn from their evil way, that I may change my mind about the disaster that I intend to bring on them because of their evil doings.
Jeremiah 26:13. Now therefore amend your ways and your doings, and obey the voice of the Lord your God, and the Lord will change his mind about the disaster that he has pronounced against you.
Jeremiah 26:19. Did King Hezekiah of Judah and all Judah actually put him to death? Did he not fear the Lord and entreat the favor of the Lord, and did not the Lord change his mind about the disaster that he had pronounced against them? But we are about to bring great disaster on ourselves!”
In all of these passages, the identical Hebrew word is used in reference to God changing his mind. Should we believe a 16th century reformer who apparently had not bothered to read the Bible, or should we believe the Bible. That is a very easy choice for me to make—and I thank God that I am free in Christ to make that choice!
All quotations from Scripture are from the NRSV unless otherwise noted.
The Hebrew verb "nâcham", has its root meaning, literally, "to draw the breath, to pity, to grieve"
When used in niphal, as in the passages that reference God, it means "to lament, to grieve, to pity", for the actions of others.
The passages that you quote, are countered by verses like,
Numbers 23:19, "God is not man, that he should lie, or a son of man, that he should change his mind. Has he said, and will he not do it? Or has he spoken, and will he not fulfill it?" (ESV)
1 Samuel 15:29, "And also the Glory of Israel will not lie or have regret, for he is not a man, that he should have regret.”
or, the NRSV which you use, "Moreover the Glory of Israel will not recant or change his mind; for he is not a mortal, that he should change his mind."
It is not as simple as "God changing His Mind", as though what He had originally planned to do, did not work out!
A good example is the Book of Jonah. God sent Jonah to proclaim a certain destruction of the city of Nineveh, the Assyrian capital, which was exceedingly wicked. However, in this Message of certain doom, God also included a Message of Hope and Deliverance. Had the people rejected what Jonah had told them from God, God would have destroyed them. However, instead, the king "saw the light", and commanded that all, from the greatest to the smallest, should turn from their sinful and wicked ways (repent), and throw themselves on the Mercy of God, which is clear in chapter 3, verses 5-9. In verse 10 we read, that, because of their actions, "And God saw their works, that they were turned from their evil way: and God had mercy with regard to the evil which he had said that he would do to them, and he did it not" (Douay-Rheims). They did according to what God had required of them, and God, in His Infinite Wisdom and Kindness and Mercy, "spared them" from their certain destruction.
This is very clear from passages like Ezekiel 18:
21 “But suppose the wicked stop doing all the sins they have done and obey all my rules and do what is fair and right. Then they will surely live; they will not die. 22 Their sins will be forgotten. Because they have done what is right, they will live. 23 I do not really want the wicked to die, says the Lord God. I want them to stop their bad ways and live.
24 “But suppose good people stop doing good and do wrong and do the same hateful things the wicked do. Will they live? All their good acts will be forgotten, because they became unfaithful. They have sinned, so they will die because of their sins.
25 “But you say, ‘What the Lord does isn’t fair.’ Listen, people of Israel. I am fair. It is what you do that is not fair! 26 When good people stop doing good and do wrong, they will die because of it. They will die, because they did wrong. 27 When the wicked stop being wicked and do what is fair and right, they will save their lives. 28 Because they thought about it and stopped doing all the sins they had done, they will surely live; they will not die. 29 But the people of Israel still say, ‘What the Lord does isn’t fair.’ People of Israel, I am fair. It is what you do that is not fair.
30 “So I will judge you, people of Israel; I will judge each of you by what you do, says the Lord God. Change your hearts and stop all your sinning so sin will not bring your ruin. 31 Get rid of all the sins you have done, and get for yourselves a new heart and a new way of thinking. Why do you want to die, people of Israel? 32 I do not want anyone to die, says the Lord God, so change your hearts and lives so you may live.
God cannot, and does not "change His Mind"; but does Act according to His own Wise Counsel, and Commands. When we humans CHOOSE what is right as Commanded by the Lord, He Acts according to His own Words, Doing what He intended in the first place. We have the "options" given by the Lord, and they all have consequences. To OBEY the Lord, or to DISOBEY the Lord, and what follows is what God had already said He would do.