Does hearing the rule: "Whatever leads to the unlawful is unlawful" establish proof against a person, such that he becomes aware of the unlawfulness of actions that lead to the unlawful, and denying the unlawfulness of these actions becomes disbelief (kufr)?

The rule "what is known by necessity to be part of the religion" is not absolute or within the grasp of the common people. Rather, it has a detailed explanation that is hidden from them. Therefore, relying on it is not sufficient to establish proof or remove ambiguity. And whoever denies a ruling, using it as an argument, does not become an apostate, because he has not denied a ruling known by necessity to be part of the religion. The meaning of "known by necessity" is that both the elite and the common people are equal in their knowledge of it, such that no doubt can arise about it, such as the number of prayers and the prohibition of fornication and alcohol.