Sorry, this is a direct quote from this page: https://www.biblegateway.com/passage/?search=Genesis+17&version=NKJV
It's from the New King James Version...
Gen 17.7 7 And I will establish My covenant between Me and you and your descendants after you in their generations, for an everlasting covenant, to be God to you and your descendants after you.
Apparently, the New King James Versions sees no conflict between the old KJV "seed" and their revised version "descendants." The "seed," singular, is a composite singular word, indicating a plural entity, such as a corporate body of believers or a corporate body of heirs.
Nations are built from a common set of descendants who certainly include those who enter into the nation from without. The product of intermarriage is a distinct nation. In the case of Israel, that nation has already been established and need not require there all be biological heirs of Abraham. The nation was built in ancient times, and its people preserved through the Jews. Only Jews may start this nation again. But non-Jews may certainly become Israeli citizens.
There are natural and spiritual descendants, meaning both Israel and a multitude of nations. But those nations, to belong to Abraham, must have Christian constitutions. There must be a genuine spiritual people among them, determining the law for the land.
The whole reason I believe in natural nations is precisely because they've been fading away. The fact they have existed at all indicates it was God's desire to have them, both Israel under the Law and Christian nations later, after Christ. God wishes to have godly nations devoted to the faith of Abraham. They are all descendants of Abraham through Christ, his "seed," singular. But that "seed" also contains a composite singularity. Christ contains both the natural descendants of Abraham, which is Israel, and the many nations who will convert to faith and adopt a Christian constitution.
There is some confusion between the idea of natural nations and the sense of being a "spiritual nation."
1) Israel is the natural descendants of Abraham. But Israel is also intended to be "spiritual," ie a nation practicing the faith and obedience of Abraham.
2) Christian nations are not the natural descendants of Abraham. They are, however, spiritual descendants of Abraham through Christ.
3) Israel and Christian nations are all spiritual descendants of Abraham through faith in Christ. They are all "called" to be nations of faith, when they appear as such. Not all nations, however, appear as such. Some have remained pagan nations.
On the other hand, those nations called to be nations of faith are not always "chosen." That is, they sometimes fail and are temporarily rejected when those who fail to have faith and fail to obey lead the nation astray.
I do have some confusion over the "called" and "chosen" ideas in Scriptures. But I realize that not everybody in a Christian nation is God's elect. The nation itself is called to be a nation of faith, but not everybody has faith.