Sam, I don’t think the Ohr Hachayim Hakadosh was being Mechadesh that everything is a Mesora. His point was that the Halachos preceded the Drashos, and that we are allowed to suggest alternative Drashos to the same Halachos.
The Rambam seems to be saying that Machlokes can only happen in Shikul Hadaas instances where something new came up. Otherwise, we say that they surely didn’t forget what the practiced Halacha was. My Kasha earlier from Smicha on Yom Tov is not Shver, since it is a Derabanan and is therefore dependent on Shikul Hadaas.