Dinei derabban and Divrei Sofrim (IV)

Getting back to our regularly scheduled program, we have been discussing R’ Elchanan’s chiddush that the mechayeiv of dinei derabbanan, the license for their enactment, is that Chazal are simply revelaing ratzon Hashem which is binding even sans tzivuy. Two further ra’ayos that REW brings:
1) In the parsha of Yehudah and Tamar, Rashi cites Chazal that the bais din of Shem prohibited various arayos. How could this enactment have been binding when the mitzvah of lo tasur had not yet been given? REW explains that it was binding as a revelation of ratzon Hashem. This ra’aya strikes me a particularly weak in light of the rishonim who understand that the mitzvah of dinim for bnei noach means establishing a justice system. If arayos are an issue that pertains to social justice, wouldn’t enactments of Shem’s bais din fall under this rubric?
Last ra'ya coming up shortly...