It is possibly true that the City which descends from Heaven is a prologue, which happens before literally before the Millennium, though, in the text, it is presented after the Millennium.
The New Jerusalem City is portrayed as coming down from Heaven with full occupancy of numbers, of all the saved who are written in the book of life. It reveals that Jesus reign is complete and he now hands over the Kingdom to the Father as in mission accomplished. The text presents it after the Millennium because the Millennium must encompass the totality of the New Covenant Age that Christ rules through his two witnesses (His Church), the anointed workmen of the harvest who are pouring out the Golden Oil of Salvation and bringing once many enemies of Christ to Christ through the preaching if the Gospel. The takeover of the Gentile nations is happening on the spiritual level, within the spiritual warfare context described in the book of Ephesians, that is being waged for the hearts and minds of those peoples from all nations that Jesus is bringing into submission to his Cross.
This is the Millennium Reign of the Cross.
I want everyone to know that the Millennium reign is the reign of Cross and is the Millennium Reign of Christ's Cross until he destroys the last enemy death and hands over the Kingdom to God the Father when he returns to call it a day by the blowing of the 7th Trumpet of God and by declaring time not longer as far as the harvest is concerned.
So the 'type' Church City on earth where the two witnesses are within the outer unmeasured court ministering the gospel to the Gentile nations who trample them in the process is the prologue to the 'anti-type' City New Jerusalem which is then revealed at the crescendo to the finale at Christ's brilliant coming.
The Millennium Reign of the Cross must be before the Second Coming at the time the last trumpet is sounded and time no longer declared.
That Mountain the rock comes from is the 'throne of God' on earth, it is the Kingdom of God, from which Jesus comes. Jesus is the one who 'smashes the nations' as 'pots of clay', just as Jesus is also that rock which smashes the the 'feet of iron mixed with clay'.
The smashing is happening within the Millennium Reign of the Cross through his Two Witnesses who are defeating the hidden enemy within a war theatre that is spiritual and one that involves spiritual weapons. The Spiritual weapons of Ephesians 6:10-20 are at the disposal of the Two Witnesses who reign down the Fire of the Holy Spirit upon the once enemies of the Cross and by bringing those once enemies from all nations, as the many subjects of the Cross, to serve Christ at the foot of His Cross. Christ is continuing to reign in this manner.
You will note that Christ is reigning within the Millennium Reign of the Cross through his Living Stones that are of the members of his body, within the outer unmeasured court of the Gentiles. The 'type' outer court Church City of the symbolic two witnesses (two olive trees - New Testament Jews and Gentiles) is a prologue to the New Jerusalem of the 'anti-type' City that descends from God and comprises of those who have been redeemed (sealed) from the earth.
Then, Jesus is revealed from Heaven, with the still blowing of the Last Trumpet, and the first resurrection happens. Only after the beast and false prophet are destroyed.
The Beast and the false prophet within the 1st-century context is the Herodian Dynasty (five kings have fallen and one (Agrippa II is) that the False Pharisaical Religious System rode to send out a flood of people from city to city, after the Church members (Two Witnesses), in order to prevent the Child (Gospel) from being spread into the world. The Beast and the False Prophet of the 1st Century must have been first prevented from persecuting the Church so that the Gospel can be spread into the many Gentile nations. Satan was chained, in other words, Satan was dispossessed of his Queen (False Prophet) and King (Herodian Beast) chest pieces.
The Millennium Reign of the Cross must, therefore, encompass the entire New Covenant Age after the Beast and the False Prophet was Slain in 70AD (The Batlle of Armageddon). The Millennium Reign of the Cross must encompass two events, where the first is the slaughter of the Beast and the False Prophet in 70AD (The Battle of Armageddon) and the last event before Christ's brilliant Coming must be when Satan is armed with the Beast of the Bottomless Pitt in a last attempt to deceive the Gentile nations for the last and final time, in the Battle of Gog and Magog.
The 1st Beast and False Prophet was the Pharisaical Religious System that was inspired by Satan to attack Christ's 1st Century Apostolic Church (read the entire Acts of the Apostles).
The successor of the Iron Legs of the Roman Empire is the 'Holy Roman Empire', which is iron, mixed in with the clay, of the 'Catholic Church'.
No, this is a complete opposite. Roman Empire had historically been the strong man that prevented the Beast of the Bottomless Pitt from rising throughout the century past, for they are the ones who inflicted the mortal wound in 70AD when they destroyed the False Prophet's Temple of Doom and the 1st Beast that protected the False Prophet back then, where abominations were being offered.