I don’t exactly agree with you about the predestination part, but I think you are correct that Satan will probably appear as a saviour, rather than a being of evil. I think his appeal will be very broad though. Many so-called Christians will believe he is Jesus, while Muslims may believe he is their Mahdi, and Jews, their Messiah.
A study of the world religions reveals that Satan was very crafty when injecting doctrine for his own purposes in all of the religions of man. I believe he has been preparing for his final “game plan” throughout all of human history, seeding everywhere false prophecies that he will be able to fulfill when the time comes. Even amongst Christians, he has managed to accomplish this. Look, for example, at how many Christians believe in the unbiblical rapture doctrine, or the false Marian apparitions that both set up just such a situation which Satan will be able to fulfill.
Paul is the one who said that 'some' were chosen/elected before the foundation of this world. This foundation as used by Paul in Ephesians is a verb that means the casting down - overthrow... and world is age. Christ used what the translators used 'foundation' as well but IIRC some were in the form of a noun. But they reference to a particular event before this flesh age.
Only those of the chosen/elect can commit the unpardonable sin. As there are those that were given the the spirit of slumber for their own protection Romans 11:8.
Some traditions of men are so entrenched in modern doctrines it is considered blasphemy to even question their Biblical standing. But hey, as Christ gave that parable of the sower, we can plant seeds of truth and only God causes the germination.