Yes, I understand what you were getting at. The reckoning of his seed is not only genealogical it would seem.
Well, I can see your point here about priesthood, but it certainly was about inheritance.
Here are some scripture teaching ideas to reckon the seed spoken of in Genesis 15.
Ro 9:7 Neither, because they are the seed of Abraham, are they all children: but, In Isaac shall thy seed be called.
Ga 3:16 Now to Abraham and his seed were the promises made. He saith not, And to seeds, as of many; but as of one, And to thy seed, which is Christ.
Heb 11:18 Of whom it was said, That in Isaac shall thy seed be called:
Gal 3:16 is especially interesting to consider. The seed which is Christ, or an annointed seed.
Annointed seed
Isaac
Jacob
Joseph
Moses
It is interesting to note what generations acts 7 has to say.
7:6 And God spake on this wise, That his seed should sojourn in a strange land; and that they should bring them into bondage, and entreat them evil four hundred years.
7 And the nation to whom they shall be in bondage will I judge, said God: and after that shall they come forth, and serve me in this place.
8 And he gave him the covenant of circumcision: and so Abraham begat
Isaac, and circumcised him the eighth day; and Isaac begat
Jacob; and Jacob begat the twelve patriarchs.
9 And the patriarchs, moved with envy, sold
Joseph into Egypt: but God was with him,
10 And delivered him out of all his afflictions, and gave him favour and wisdom in the sight of Pharaoh king of Egypt; and he made him governor over Egypt and all his house.
11 Now there came a dearth over all the land of Egypt and Chanaan, and great affliction: and our fathers found no sustenance.
12 But when Jacob heard that there was corn in Egypt, he sent out our fathers first.
13 And at the second time Joseph was made known to his brethren; and Josephs kindred was made known unto Pharaoh.
14 Then sent Joseph, and called his father Jacob to him, and all his kindred, threescore and fifteen souls.
15 So Jacob went down into Egypt, and died, he, and our fathers,
16 And were carried over into Sychem, and laid in the sepulchre that Abraham bought for a sum of money of the sons of Emmor the father of Sychem.
17 ¶
But when the time of the promise drew nigh, which God had sworn to Abraham, the people grew and multiplied in Egypt,
18 Till another king arose, which knew not Joseph.
19 The same dealt subtilly with our kindred, and evil entreated our fathers, so that they cast out their young children, to the end they might not live.
20
In which time Moses was born, and was exceeding fair, and nourished up in his fathers house three months: {exceeding fair: or, fair to God}
21 And when he was cast out, Pharaohs daughter took him up, and nourished him for her own son.
22 And Moses was learned in all the wisdom of the Egyptians, and was mighty in words and in deeds.
23 And when he was full forty years old, it came into his heart to visit his brethren the children of Israel.
24 And seeing one of them suffer wrong, he defended him, and avenged him that was oppressed, and smote the Egyptian:
25
For he supposed his brethren would have understood how that God by his hand would deliver them: but they understood not.
So here we have
Isaac, Jacob, Joseph, Moses......