You are missing the full context of Genesis 15:1-6 as this is speaking about the seed covenant God made with Abraham about he and Sarah being childless. What God gave in showing Abraham all the stars in heaven God said, Look now toward heaven, and tell the stars, if thou be able to number them: and he said unto him, So shall thy seed be. From that day forward all who are of faith are the seed of faithful Abraham.
Paul also confirms this in Gal 3:29 And if ye be Christ's, then are ye Abraham's seed, and heirs according to the promise.
The Millennium does not start until Christ returns as at that time we who are His own are to meet Jesus in the air, Matthew 24:29-31; 1Thessalonians 4:13-18.
Sorry, but in the Millennium Kingdom no one is married or given in marriage. There will be no procreation to further any new generations.
Mat 22:30 For in the resurrection they neither marry, nor are given in marriage, but are as the angels of God in heaven.
I only have time to answer the first part of your reply now. God willing I'll answer the second part soon later.
I will concede the literal interpretation of Genesis 15:5 can refer to the many nations on earth before the millennial kingdom (Rom. 4:18)
But your claim procreation doesn't occur during the millennial kingdom is incorrect. I agree believers will have resurrection bodies like Christ's and they will not marry or be given in marriage, BUT they are Kings and Priests in Christ's kingdom and they evangelize those who enter the kingdom unsaved AFTER Armageddon.
9 And they sang a new song, saying: "You are worthy to take the scroll, And to open its seals; For You were slain, And have redeemed us to God by Your blood Out of every tribe and tongue and people and nation,
10 And have made us kings and priests to our God; And we shall reign on the earth." (Rev. 5:9-10 NKJ)
The "survivors of Armageddon" were neither "sheep or goats" (who like the virgins and servants in the same context [Matthew 25:1-46] heard enough of the gospel to be liable to its judgment).
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They may number in the billions, but they enter Christ's kingdom still alive with their normal human bodies. But aging has stopped by God's command:
20 "No more shall an infant from there live but a few days, Nor an old man who has not fulfilled his days; For the child shall die one hundred years old, But the sinner being one hundred years old shall be accursed.
21 They shall build houses and inhabit them; They shall plant vineyards and eat their fruit.
22 They shall not build and another inhabit; They shall not plant and another eat; For as the days of a tree, so shall be the days of My people, And My elect shall long enjoy the work of their hands.
23 They shall not labor in vain, Nor bring forth children for trouble; For they shall be the descendants of the blessed of the LORD, And their offspring with them.
24 "It shall come to pass That before they call, I will answer; And while they are still speaking, I will hear.
25 The wolf and the lamb shall feed together, The lion shall eat straw like the ox, And dust shall be the serpent's food. They shall not hurt nor destroy in all My holy mountain," Says the LORD. (Isa. 65:19-25 NKJ)
Billions likely survive Armageddon and God doesn't slaughter them. Those who never heard the gospel sufficient to make them liable for not obeying it (as was not about the virgins, servants and sheep and goats Mt. 25:1-46 who failed to be ready for the King's return).
These unsaved billions are evangelized during the millennial kingdom and then put to one final test at the end of that kingdom:
4 And I saw thrones, and they sat on them, and judgment was committed to them. Then I saw the souls of those who had been beheaded for their witness to Jesus and for the word of God, who had not worshiped the beast or his image, and had not received his mark on their foreheads or on their hands. And they lived and reigned with Christ for a thousand years.
5 But the rest of the dead did not live again until the thousand years were finished. This is the first resurrection.
6 Blessed and holy is he who has part in the first resurrection. Over such the second death has no power, but they shall be priests of God and of Christ, and shall reign with Him a thousand years.
7 Now when the thousand years have expired, Satan will be released from his prison
8 and will go out to deceive the nations which are in the four corners of the earth, Gog and Magog, to gather them together to battle, whose number is as the sand of the sea.
9 They went up on the breadth of the earth and surrounded the camp of the saints and the beloved city. And fire came down from God out of heaven and devoured them.
10 The devil, who deceived them, was cast into the lake of fire and brimstone where the beast and the false prophet are. And they will be tormented day and night forever and ever.
(Rev. 20:4-10 NKJ)