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This is the question that interests me the most. Is there anything in the OT or anywhere else that tries to explain why it was necessary for Jesus to be executed in order for him/God to "save" us?
Somehow it works like this. If you break the Law, you'll be sentenced, whether you understand Law or not, whether you read it or not, whether you ever study law itself or not. That's the case.
Similarly God's Law is embedded in your heart and soul. If you offended His Law, you'll be judged and put to jail. If you sinned you'll be driven out of His kingdom, the same as how Adam sinned and being driven out of Eden, and same as how Lucifer the archangel sinned and being driven out of heaven.
God is to save souls instead of bodies here on earth. Here on earth humans are basically not in His Kingdom. That's why the myseries. And everyone requires to pass the final judgment in order to return to God and His kingdom in heaven. And God foresaw that you won't stand a chance in the judgment. Jesus Christ is thus sent and a new covenant is in place which allows you to escape from the Judgment by Law.
If you refuse to take the new covenant, and when you are judged to be sinful, a permanent separation from God will be called. It is said that His sheep will take the new covenant. It is also said that those who aren't His sheep will continue to sin against His Law without repentance.
Moreover, it is said in the Book of Revelation that Satan (who is said to be an ex-archangel) is an accuser accussing people of sinning in order to keep them in jail with him. So good luck, either to find yourself a good lawyer or prepare yourself for the defense.
Revelation 12:10
Then I heard a loud voice in heaven say: "Now have come the salvation and the power and the kingdom of our God, and the authority of his Christ. For the accuser of our brothers, who accuses them before our God day and night, has been hurled down."
Alternatively, take the Second Covenant as a gift such that the final Judgment by Law can be avoided.
Moreover, your OP is more of a question of how pre-destination is implemented. By far in human history, no one knows. Calvin ever made an attempt and Calvinism which is with flaws and not necessarily true.