Two people had custody before the cataclysm. One went through the cataclysm which eliminated anyone and any other records. It was then a very short time before the recitation was a part of a tribal identity. What is speculative about that?
What is speculative is ... well everything. The Bible says
nothing about a chain of custody. So instead of speculating, why not start with what we
do actually know:
We know that neither Adam, Noah, Abraham nor Joseph wrote Genesis.
We know that a lot of the material in Genesis aligns well with the ancient near eastern culture, ancient views of the cosmos, covenants, slavery, etc.
We know that the material in Genesis 1-11 bears a many similarities to other ancient near eastern texts (e.g. Egyptian and Babylonian creation accounts, Gilgamest epic, etc) but there are also many key differences which sets it apart and presents the text as a polemic against the beliefs of Israel's neighbours.
The genre of Genesis 1 is not a straight-forward historical account, but contains a lot of literary devices - as I have quoted elsewhere "
Genesis 1 … is not written in the style we normally associate with historical report. The original Hebrew of this passage is marked by intricate structure, rhythm, parallelism, chiasmus, repetition and the lavish use of number symbolism. These features are not observed together in those parts of the Bible we recognize as historical prose."
These things matter; they are part of the context of the text. When you ignore them and try to put them in a different context (e.g. modern day scientific context) you will end up with confused ideas that simply don't work.