I don’t disagree with anything you said.
You would do well not to confuse casual internet parlance with deep (and more precise) theological discussion.
The “source” for the non-Torah versions would, of course, be eye-witness accounts passed down trhough generations of the same event dictated to Moshe by HaShem. The two routes are not inconsistent.
I apologize if I misunderstood you, but it seems to me you were saying both that the Torah and the other stories had a common source, and that the Mabbul was a local event.
Anytime one is discussing HaShem or the Torah, it is best to be as precise as possible. Casual Internet parlance can lead to all sorts of assumptions by readers.