The division occurs immediately,
The being cast into the LOF doesn't. That is over a thousand years after Christ returns.
Ar his return, he tells the goats to depart from him into the fire reserved for satan and his angels (vs 41).
No, that is not what happens when he returns. It happens after satan is imoprisoned, after a thousand years of the rod of iron rule over the nations, after satan is released and bring an army to Jerusalem, after that army is burned by fire from God the Father, after the second resurrection of "the rest of the dead", after the Great White Throne Judgement (GWTJ) only Nat that time will they be cast into that fire.
Then explain this,
And to you who are troubled rest with us, when the Lord Jesus shall be revealed from heaven with his mighty angels, In flaming fire taking vengeance on them that know not God, and that obey not the gospel of our Lord Jesus Christ: Who shall be punished with everlasting destruction from the presence of the Lord, and from the glory of his power; When he shall come to be glorified in his saints, and to be admired in all them that believe (because our testimony among you was believed in that day. 2Thes.1:7-10
Just because something is written a sentence or two away does not equal a short period of time. Revelation explains that there is a huge amount of time between the first resurrection and the second one.
A massive amount of people die from fire because, "the Lord Jesus shall be revealed fromheaven with his mighty angels, In flaming fire".
He comes in flaming fire not using flaming fire. All it means is he comes very fast like how fast flashing lightning is. In zero second coming passages does Christ ever use fire against his enemies. He uses the Word which is symbolized as a sword from his mouth.
the day of the Lord will come as a thief in the night; in the which the heavenss hall pass away with a great noise, and the elements shall melt with fervent heat, the earth also and the works that are therein shall be burned up. 2Pet.3:10
That happens after the Great White Throne Judgement (GWTJ) and LOF second death, seen in Revelation 21. There are no mortals on the planet when the NHNE happens and the old are gone.