There seems to be a machlokes between the Bavli and Yerushalmi whether a mitzvah given pre-mattan Torah is a stronger mitzvah or a weaker mitzvah. The Yerushalmi quoted in Tos Kid 38 writes that that the mitzvah of matzah is not doche the issur of chadash because matzah was given pre-mattah Torah and therefore is a weaker aseh. However, the Bavli Yevamos 5b in searching for a source that an aseh can be doche even an issur kareis says you cannot bring proof from the korban pesach being doche shabbos, or milah being doche Shabbos, because these mitzvos were given pre-mattan Torah and therefore are stronger than all other mitzvos.
How does either possibility fit with the Rambam's view (see last post) that all mitzvos are binding only because they were given at Sinai? There is no pre-mattan Torah mitzvos -- everything became binding at the same time. Why should the fact that there is some pre-mattan Torah background history to some mitzvos have any legal ramification?