“The Mishna Brura rules (his source is Sefer Hachinuch almost word for word, but without attribution) that apikursos includes any opinion that is contrary to Daas Torah. Meaning, even if you follow all Halachos, if you have an opinion that conflicts with that of the Torah, you violate this laav.”
Problem with this paragraph:
The Sefer Hachinuch writes in mitzvah 387: ???? ??? ?? ??????? ??? ???? ????????? ????? ????? ??? ????
???? ?????? ????? ????
??? ????? ???? ???? ?? ??????
The Mishnah Berurah in the second Beiur Halacha quotes this and contrary to what you said, he does attribute it to the Sefer Hachinuch. The problem is that he doesn’t quote it accurately. He is listing the six constant mitzvos and the sixth one is this one. He changes the words though and writes: ????? ????????? ??? ?? ?????? ???? ??? ????
??? ?????
And even with the Beiur Halacha’s version, it still has nothing to do with whether you can pasken agadda/hashkafa.