I believe this will be related to a similar question about a statement by Rabbi Sacks. Rabbi Jeremy Wieder said in a lecture titled Non Literal Interpretation of Scripture in Jewish Tradition (and elsewhere as I understand it) that based on principles mentioned by the Rambam and Rav Saadia Gaon that, in simple terms, you generally understand the plain meaning of the text except when reason doesn't allow such a literal understanding, provided doing so doesn't violate core principals of Judaism. One example he gives is that while the verse speaks of God creating a rainbow, upon being shown that a rainbow is a natural property of optics and refraction, Rambam reinterpreted it to mean that God simply used a rainbow as a sign.
However, he goes on to say that the whole of the first 11 chapters could be understood to be like a parable, not something that actually happened. (Perhaps for details like halachos based on this portion he would take a modified approach, not sure.) (Also he says it wouldn't be heresy to say the Avos never existed but he would personally be very uneasy about taking it that far. Basically he said the only thing that you must understand as literal was the revelation at Mt. Sinai.)
So my question is, is there any precedent for that position on such a large scale? It's much more than understanding the nature of a rainbow. It seems to go against all of Chazal's commentaries and Jewish tradition from Mt. Sinai to say that the events didn't happen. To be sure, if correct, his approach would basically answer my other question that asks for a reconciliation between science that seems to say human history goes well beyond 6,000 years ago and that it proceeded uninterrupted straight through the Mabul (as well as another detail I didn't ask about that the history of languages shows they developed with no dispersion at a tower 4,000 years ago). But the halachic basis for such a liberal approach seems questionable to me, hence the question.
EDIT: I added a bounty to try to draw more eyes to the answers and get some other people's feedback on them or improved answers so I can get help choosing one for the accepted answer.