God's promise to Abraham was that he would have descendants as numerous as the Stars, what he did not promise is that they would all be his chosen people only that Isaac would.
All Scripture is quoted from the King James translation unless otherwise noted.
Genesis 12:7 And the LORD appeared unto Abram, and said, Unto thy seed will I give this land: and there builded he an altar unto the LORD, who appeared unto him.
This is not the promised land it is the middle east, (or what is now the Arabic nations), this is the promise God made to Abraham:
Genesis 15:1 through 6 After these things the word of the LORD came unto Abram in a vision, saying, Fear not, Abram: I am thy shield, and thy exceeding great reward. And Abram said, Lord GOD, what wilt thou give me, seeing I go childless, and the steward of my house is this Eliezer of Damascus? And Abram said, Behold, to me thou hast given no seed: and, lo, one born in my house is mine heir. And, behold, the word of the LORD came unto him, saying, This shall not be thine heir; but he that shall come forth out of thine own bowels shall be thine heir. And he brought him forth abroad, and 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. And he believed in the LORD; and he counted it to him for righteousness.
(This should also answer your other question concerning Isaac)
Genesis 15:13 through 16 And he said unto Abram, Know of a surety that thy seed shall be a stranger in a land that is not theirs, and shall serve them; and they shall afflict them four hundred years; And also that nation, whom they shall serve, will I judge: and afterward shall they come out with great substance. And thou shalt go to thy fathers in peace; thou shalt be buried in a good old age. But in the fourth generation they shall come hither again: for the iniquity of the Amorites is not yet full.
Even though this was many years before Jacob became Israel it is clearly the Nation of Israel that God is referring to. This not only shows that God is precognizant of events, but that he has a particular path upon which he intends his blessings to flow.
As for showing compassion to Hagar let's take a look at that:
Genesis 16:7 through 13 And the angel of the LORD found her by a fountain of water in the wilderness, by the fountain in the way to Shur. And he said, Hagar, Sarai's maid, whence camest thou? and whither wilt thou go? And she said, I flee from the face of my mistress Sarai. And the angel of the LORD said unto her, Return to thy mistress, and submit thyself under her hands. And the angel of the LORD said unto her, I will multiply thy seed exceedingly, that it shall not be numbered for multitude. And the angel of the LORD said unto her, Behold, thou art with child, and shalt bear a son, and shalt call his name Ishmael; because the LORD hath heard thy affliction. And he will be a wild man; his hand will be against every man, and every man's hand against him; and he shall dwell in the presence of all his brethren. And she called the name of the LORD that spake unto her, Thou God seest me: for she said, Have I also here looked after him that seeth me?
By that last statement it is evident that she has not served God, since she is asking why are you looking out for me, have I served you in some way I am not aware of? Also sending her back to be subjected to Sari after the way Sari treated her doesn't seem much like compassion to me.
Genesis 16:3 through 6 And Sarai Abram's wife took Hagar her maid the Egyptian, after Abram had dwelt ten years in the land of Canaan, and gave her to her husband Abram to be his wife. And he went in unto Hagar, and she conceived: and when she saw that she had conceived, her mistress was despised in her eyes. And Sarai said unto Abram, My wrong be upon thee: I have given my maid into thy bosom; and when she saw that she had conceived, I was despised in her eyes: the LORD judge between me and thee. But Abram said unto Sarai, Behold, thy maid is in thy hand; do to her as it pleaseth thee. And when Sarai dealt hardly with her, she fled from her face.
So it was Sari who caused Hagar to be cast out into the wilderness to start with, and that after setting up Hagar to bear Abraham's child in her place since she could not have children of her own.
Genesis 16:1 and 2 Now Sarai Abram's wife bare him no children: and she had an handmaid, an Egyptian, whose name was Hagar. And Sarai said unto Abram, Behold now, the LORD hath restrained me from bearing: I pray thee, go in unto my maid; it may be that I may obtain children by her. And Abram hearkened to the voice of Sarai.
The descendants of Ishmael are the Arabic Nations, the descendants of Esau are the Palestinians, and the descendants of Jacob is the Nation of Israel: and this is the fulfillment of the covenant God made with Abraham. There was no promise made to Hagar, and matter of fact she was always harshly treated by her mistress. So I fail to understand why you feel that God had compassion on Hagar.