Strangely enough, the same basic story is told thrice, twice about Sarah and once about Rebeccah (Genesis 12; 20; 26) -- and only in the middle one does the redactor think of telling us, "btw, she was really his (half-)sister"... as if he perceived a more serious moral problem in having Abraham lying than being engaged in an (anachronistically according to the time of the story) "unlawful marriage".
Which leads me to Dagney's very insightful remark:
I realized that for every absolute law/rule for something, there was a perfectly acceptable situation in the opposite. It's all through the book.
What if the "counter-stories" were, in part, made for this very purpose, i.e. subverting the "official Torah" in the making? The story of Ruth, for instance, is almost perfectly opposite to the exclusivism of Deuteronomy. Many details of the Patriarchal stories are, too.
Edit to add: great to see you Gumby!