Satan is locked up so as not to deceive the nations. It is only at the end of the thousand years he is unlocked and deceives the nations. And the chastening those nations get for being involved in that act are disciplined in an instant rather than a long drawn out process.
Rev 20:6-7 Happy and holy is he who is having part in the former resurrection! Over these the second death has no jurisdiction, but they will be priests of God and of Christ, and they will be reigning with Him the thousand years." (7) And whenever the thousand years should be finished, Satan will be loosed out of his jail."I think not because when satan is loosed out of jail the sinning starts again. And therefore (at least some) chastening is needed.
So, the chastening which is eonian in Matthew 25:46 does not make eonian mean for the age and beyond.
Is it this verse that you mean when you wrote 'in an instant' ?Revelation 20:9
And they went up on the breadth of the earth, and compassed the camp of the saints about, and the beloved city: and fire came down from God out of heaven, and devoured them.
That looks a bit like annihilation.
The armies are crushed in an instant. That's clear from the verse. A few verses later all are judged. That includes the people from the armies. Then they wicked are cast unto LoF. Then for me there are only a few further possible 'actions'
a] It's an ET type of fire
b] They die after a while (but dead is already destroyed)
c] A period of chastening starts.