Answer

Indeed, the authority of a community rabbi comes from the fact that the community accepted him upon itself (under contract law). Therefore, in principle, a person cannot simply retract, because he is bound by a contract. However, he does have the right to leave the community, and then he is no longer bound by the contract. But as long as he remains in the community, there is no way to violate the contract. Leaving the community is not just an inward decision; it involves practical steps (otherwise the contract is meaningless): not praying in the synagogue and not participating in community events—that is, not benefiting from what it provides to its members.
But it should be noted that a rabbi’s authority applies to public questions (what to do in the synagogue or at public events), not to how a person conducts himself in his own home. There, in my opinion, he has no authority, and you may do what you think is right.
You can disagree with the rabbi and express a different position, and certainly it is even worthwhile to try to persuade him (respectfully), but at the end of the day, what he determines is what is binding.
It is true that I am not a formalist, and therefore when the rabbi goes too far there is definitely room to deviate from his directives, certainly when this is the view of the community as a whole (and not just your own view).
As for leaving Judaism, that is a different matter. Here the contract is with the Holy One, blessed be He, and therefore according to most opinions you cannot opt out of it. In principle, He has the authority to compel you even without your expressing consent, except that He chose not to make use of that authority. True, there were medieval authorities who held that one can leave Judaism, and therefore they regarded an apostate as a gentile. But that was not accepted as the halakhic ruling.