the prophecy of
Genesis 15:13 is really three prophecies
Kudos to you for meditating on the Word in this manner and for articulating your thoughts on this obvious conundrum in the scriptures, but Hank is right, you should have left off any connection of "myth" with scripture, but I'll forgive you for it.
My thoughts on the conundrum:
The "giving of the law":
5 Thou shalt not bow down thyself unto them, nor serve them, for I Jehovah thy God am a jealous God, visiting the iniquity of the fathers upon the children, upon the third and upon the fourth generation of them that hate me, Ex 20
'Visiting the father's sin unto the fourth generation' seems to be an explanation to the Exodus generation concerning their bondage BECAUSE of what their fathers did to Joseph in betraying him and selling him into bondage. AND it was precisely the fourth generation that came out of Egypt:
16 And these are the names of the sons of
Levi according to their generations: Gershon, and Kohath, and Merari; and the years of the life of Levi were a hundred thirty and seven years.
17 The sons of Gershon: Libni and Shimei, according to their families.
18 And the sons of
Kohath: Amram, and Izhar, and Hebron, and Uzziel; and the years of the life of Kohath were a hundred thirty and three years.
19 And the sons of Merari: Mahli and Mushi. These are the families of the Levites according to their generations.
20 And
Amram took him Jochebed his father`s sister to wife; and she bare him Aaron and
Moses: and the years of the life of Amram were a hundred and thirty and seven years. Ex 6
The fourth generation came out of Egypt just as it was precisely predicted three verses down from your OP passage:
13 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;
16 And in
the fourth generation they shall come hither again..... Gen 15
IMO, you're probably on the right track with your explanation of the 400 year time element of the prophecy.