In Genesis 15 why did God feel it was necessary for Him to make a solemn covenant/promise to Abram? The answer is in the Chapter, which must be read as a whole, to see it.
Genesis 15: -
After these things the word of the Lord came to Abram in a vision, saying, "Do not be afraid, Abram. I am your shield, your exceedingly great reward."
But Abram said, "Lord God, what will You give me, seeing I go childless, and the heir of my house is Eliezer of Damascus?" Then Abram said, "Look, You have given me no offspring; indeed one born in my house is my heir!"
And behold, the word of the Lord came to him, saying, "This one shall not be your heir, but one who will come from your own body shall be your heir." Then He brought him outside and said, "Look now toward heaven, and count the stars if you are able to number them." And He said to him, "So shall your descendants be."
And he believed in the Lord, and He accounted it to him for righteousness.
Then He said to him, "I am the Lord, who brought you out of Ur of the Chaldeans, to give you this land/{earth} to inherit it."
And he said, "Lord God, how shall I know that I will inherit it?"
So He said to him, "Bring Me a three-year-old heifer, a three-year-old female goat, a three-year-old ram, a turtledove, and a young pigeon." Then he brought all these to Him and cut them in two, down the middle, and placed each piece opposite the other; but he did not cut the birds in two. And when the vultures came down on the carcasses, Abram drove them away.
Now when the sun was going down, a deep sleep fell upon Abram; and behold, horror and great darkness fell upon him. Then He said to Abram: "Know certainly that your descendants will be strangers in a land that is not theirs, and will serve them, and they will afflict them four hundred years. And also, the nation whom they serve I will judge; afterward they shall come out with great possessions. Now as for you, you shall go to your fathers in peace; you shall be buried at a good old age. But in the fourth generation they shall return here, for the iniquity of the Amorites is not yet complete."
And it came to pass, when the sun went down, and it was dark, that behold, there appeared a smoking oven and a burning torch that passed between those pieces. On the same day the Lord made a covenant with Abram, saying:
"To your descendants I have given this land, from the river of Egypt to the great river, the River Euphrates — the Kenites, the Kenezzites, the Kadmonites, the Hittites, the Perizzites, the Rephaim, the Amorites, the Canaanites, the Girgashites, and the Jebusites."
In Genesis 15 we see that Abram doubted God twice.
First off Abram doubted that his reward would be great since he was fatherless and that the heir would not be of his own flesh and blood. But when God told Abram that his descendants would be of such number that it would be impossible to count them like the stars in Heaven, Abram Believed and it was accounted to him as righteousness.
In the second instance, Abram doubted that he would inherit the Earth as God promised him.
(Aside: - In verse 7 the word translated as land is the same Hebrew word found in Genesis 1:1: - “First God created the heavens and the
earth.” The tradition has been that this Hebrew word is translated as “land”, probably because of the land promise at the end of the chapter, but this is inconsistent with other passages in the Bible, such as: -
Daniel 7:27a: - “Then the kingdom and dominion,
And the greatness of the kingdoms under the whole heaven,
Shall be given to the people, the saints of the Most High.
Daniel 7:27a suggests that the Saints, Abraham’s descendants, the righteous of God, will be given dominion over the whole earth.
God’s response to Abram’s doubt was to enter into a solemn “sign” covenant, such that when Abram’s descendants understood that God had given them the land promised in Genesis 15:17-21 that they would then also know that God’s promise to Abram that he and his descendants would also inherit the whole earth will also come to pass.
Sadly, because of the fixation on the “land,” we have lost sight of god’s promise to Abram that the Saints {i.e. all the judged righteous descendants of Abraham} will, at the end of the Age of the Ages, inherit the whole earth.
Now Abiding, the first red highlighted sentence is true. God’s solemn sign covenant of Genesis 15 is unconditional. When Abraham descendants realise that God has indeed fulfilled His solemn sign covenant, that they would know that their inheritance is indeed the whole earth, not just a tiny part of it if they are truly righteous descendants of Abraham, i.e. the natural and the grafted vines.
However, your second red highlighted sentence does not ring true to me. It seems to be based on a wrong premise. In the parable of the Judgement/separation of the Sheep from the Goats, Matthew 25:31-46, both the Sheep and the Goats knew of the promised inheritance for God’s Saints. Both the sheep and the goats called Jesus, Lord, Lord but only the Sheep were judged as being righteous and entered in to receive the promised inheritance.
It seems to me, from the little you have presented, that the writer does not have this understanding.
Shalom