Ah, the real face of Calvinism! Sodom was not destroyed because God wanted it, but because it was wicked. He would have spared it had he found 10 righteous people in it. He did so with Ninevah,
And God saw their works (oh, that can't be!) that they turned from their evil way, and God repented of the evil, that he had said would do unto them and he did it not(Jonah.3:10)
How about that, a 'decree' that got overturned because a whole city repented when told they were going to be destroyed.
God gets no pleasure at of destroying the wicked,
And should I not spare Ninevah, that great city, wherein are more than sixscore thousand persons that cannot discern between their right hand and their left hand and also much cattle.(Jonah,4:10)
Jonah was a 'good' Calvinist. He was depending on that decree being carried out! God, however cares about those He has created, even the wicked, hoping they will repent (2Pet.3:9)
For the sake of discussion, I could even grant your ideas about God "changing His mind", and yet you still cannot escape the Fact that God, who has designed the human soul, knows "how much" Grace and "what kind" of Grace will be sufficient to bring any man to Repentance.
And had God seen fit to Predestine the people of Tyre and Sidon to Repent and be Saved, He could have used the Grace of Miracles to bring about their Repentance. In fact, Jesus says it is certain that they would have Repented, had God seen fit to display such miracles.
But God did not Purpose to predestine them to Repentance and Salvation, but rather to predestine them to Non-Repentance and Damnation. And so, he selected the Option (NON-performance of miracles) which would pre-determine the decision of Tyre and Sidon to NOT Repent, and be Damned.
And you have acknowledged that He knew what He was doing, what each Option entailed, and that He deliberately selected this option, and that their Non-Repentance was thus the pre-determined result of His decision, just as He foresaw.
As I said -- give Tyre and Sidon all the free will and natural moral competence you like, and you still cannot escape from the fact that God, by His election, absolutely predestined their NON-Repentance unto Damnation. Oh, we can wave our hands and suppose that God would have changed His mind if they had repented, but He knew for a fact that they would not -- for He knew that they would Repent given the performance of equivalent miracles to those performed in Chorazin and Bethsaida, but He did not purpose to predestine their Repentance, but rather to predestine their Non-Repentance unto Damnation. This is what you are desperately trying to avoid... and you can't.