Your post had me studying Isaiah 65:20. I think that scripture is saying that if a person dies at the age of 100, they died in their youth.Let's assume that the life span of each generation during the Millennium is 100 years (Isa 65:20). Which means 10 generations within 1,000 years. While the first generation may be fully saved, the last generation may not. Therefore they will be deceived by Satan.
As to the "elect" (the saved ones), they cannot be deceived.
For there shall arise false Christs, and false prophets, and shall shew great signs and wonders; insomuch that, if it were possible, they shall deceive the very elect. (Mt 24:24)
I could be wrong, thank you for this study.