There is no theological flaw. You need to reason it rationally, using logical proofs. (Even God said, come let us reason together.) If the Calvinistic definition of the elect is that God chose whom He would save before the foundation of the world, then is it possible, given the definition of God Himself, for any of them not to be saved? If it is not possible for anyone whom God has chosen to declare justified, through the way He made in His own Son, to escape God, can they, using sound reasoning, sin the unpardonable sin? We do have a thought process today that has thrown out objectivity, rationality, and sound reasoning. It has invaded the church, and has found a home in "libertarian" free will and action. Your Arminian thought process complicates things A LOT. On one hand, the Calvinistic belief is that one who has been chosen by God to be saved, cannot undo that, because it is God we are talking about. They will not sin the unpardonable sin by the grace, mercy, and faithfulness of God, which you seem to ignore. The choice of those who commit the unpardonable sin is BOUND. They are slaves of sin, and cannot choose to be righteous. For even God said that our righteousness is as filthy rags...in other words, not really righteousness. So, in God's foreknowledge/foresight, was He looking for people wearing filthy rags, or for those people who were wearing filthy rags?
Jesus spoke of the unpardonable sin in condemnation of those religious leaders who were committing it right at that moment. It had nothing to do with elect or non-elect, but the condemnation of those who at that moments damned themselves to hell with no hope of salvation, as clearly explained by the Author of salvation. This He did before the people, who can heed what Jesus said, or not heed what He said. Don't forget
context means everything.
The last question by the OP is not even worth mentioning, but I will anyway, "How could God forgive a sin which Christ has not atoned for?" Reread the context and you will learn that God WILL NOT forgive/pardon anyone committing the unpardonable sin, and it isn't because He can't, it is because He won't. As they aren't elect, Jesus sacrifice does not atone for their sins. It could, but Christ's sacrifice is the ground floor for Salvation, and so if they aren't saved, they aren't atoned for. Universalism is a heresy.