Could the Millennium Reign start without The Return of Physical CHRIST?
No. He has to return before the Reign over the nations begins:
Christ will exercise his Kingly authority on Earth when the 7th trump sounds:
Rev 11:15 And the seventh angel sounded; and there were great voices in heaven, saying,
The kingdoms of this world are become the kingdoms of our Lord, and of his Christ; and he shall reign for ever and ever.
The rod of iron reign only begins after Christ has come:
Rev 2:25 But that which ye have already
hold fast till I come.
Rev 2:26 And he that overcometh, and keepeth my works unto the end,
to him will I give power over the nations:
Rev 2:27
And he shall rule them with a rod of iron; as the vessels of a potter shall they be broken to shivers: even as I received of my Father.
The saints reign will be on the Earth not from heaven:
Rev 5:9 And they sung a new song, saying, Thou art worthy to take the book, and to open the seals thereof: for thou wast slain, and hast redeemed us to God by thy blood out of every kindred, and tongue, and people, and nation;
Rev 5:10 And hast made us unto our
God kings and priests: and we shall reign on the EARTH.
second coming:
Rev 19:15 And out of his mouth goeth a sharp sword, that with it he should smite the nations: and
he shall rule them with a rod of iron: and he treadeth the winepress of the fierceness and wrath of Almighty God.
After the second coming:
Rev 20:4 And I saw thrones, and they sat upon them, and judgment was given unto them: and I saw the souls of them that were beheaded for the witness of Jesus, and for the word of God, and which had not worshipped the beast, neither his image, neither had received his mark upon their foreheads, or in their hands; and
they lived and reigned with Christ a thousand years.
All these prove a literal kingdom with Christ and his saints physically here on the Earth literally ruling over human beings of various nations.