Well, notice that IN HEAVEN there were angels with the freedom to reject God’s sovereignty and were then fallen with satan ... also, during the millennial reign of Christ, with satan bound in the pit, some of the ‘elect’ use their free will to turn in the wrong direction, running on their pride of self ...
The angels falling looks to be a one-time event prior to the beginning, and there is no redemption for those that fell. We do not see an ongoing struggle in Heaven, just worship.
As for the ‘elect’ in the millennium, you would have to point me to the passage, so I can understand your point.