Maybe we can compare what the bible says. The bible says:
10 When God saw what they did and how they turned from their evil ways, he had compassion and did not bring upon them the destruction he had threatened.
Jonah’s Anger at the Lord’s Compassion
4 But Jonah was greatly displeased and became angry. 2 He prayed to the Lord, “O Lord, is this not what I said when I was still at home? That is why I was so quick to flee to Tarshish. I knew that you are a gracious and compassionate God, slow to anger and abounding in love, a God who relents from sending calamity.
You make a case for God
not changing his mind, not based upon what the bible says (unless you have some verse to support it), but upon history? There could be any number of reasons why ninevah was destroyed later by the Medes - God's fulfillment of this prophecy (which it clearly says he had relented from) is probably not one of them.