A few points in answer to the question.
Definition of heresy might differ from definition of truth. Can one be a heretic for denying a false belief? I think so. Muslims would deem a heretic one who denies Muhammed as a true prophet, and a person might be correctly deemed a heretic under these rules. Similarly, perhaps a person can be deemed a heretic in Judaism for denying kabbalah, even if kabbalah was made up. If so, a person should be brave enough to be a technical heretic yet intellectually honest.
A person might have a wrong-headed belief that is not shared by any major Gadol, but that might make him a misguided soul or a fool, rather than a heretic. I might believe in UFOs or that the government is controlling me via microwave radiation, but that doesn't make me a heretic.
The purported shalshelet hakabbalah, establishing the masorah, seems to be messed up or fictionalized. See here. Disallowing someone who has the methodology to realize this from concluding this under heresy, because major Gedolim would not have similar methodology to reach the same conclusion, is an effective way of bolstering a problematic masorah. This may not be the intent, but it is an effect.
Masechet Horayot addresses the possibility of all Israelites following a mistaken ruling by the Sanhedrin. One is forbidden from following a ruling he knows to be wrong, under an incorrect application of lo tasur.
The Rambam did not include kabbalah in his list of required beliefs. However, R’ Tzadok haKohen writes in his Sefer Zichronos, citing a tshuvah of the Bach:
תה שנתפרסמה חכמת האמת בעולם מוסכם בפי חכמי ישראל האמיתים וכל הכופר בה הוא מכלל האפיקורסים ...דהמלעיג על דברי חכמים ומדבר דופי על דברי הקבלה שהיא מקור התורה ועיקרה וכולה יראת שמים פשיטא שאין לך מזלזל בדברי חכמים גדול מזה
Adding to Ikarei Emunah is not something new. The Divrei Chaim made the belief, that the Ohr HaChaim commentary on Chumash was written with ruach hakodesh, mandatory. As well as following Shulchan Aruch, since it was written with Ruach Hakodesh. The
משנה הלכות in 7:160 extended this to Mishna Berura:
It is obvious that someone who lacks ruach hakodesh is not able to composes a holy work such as the Mishne Berura. If he doesn’t believe that the Mishne Berura was written with ruach hakodesh then he is an apikorus and denier of God’s Torah.
But that does not mean that everyone agrees to this position, that there is an extension of ikkarei emunah.