Ismailis in Pakistan

First of all, you are ABSOLUTELY wrong about this being the unanimous decision among all Muslims since the time of Holy Prophet (SAW). On the contrary, there have been a number of Ulema and Aulia who have believed otherwise. I found this clip on YouTube about Maulana Qasim Nanotvi (founder of Deobandi school of thought):

Well, I’d argue that Jamaat Ahmadiyya, for one, was such a mainstream denomination, until you declared them non-Muslims. It’s a great move on your part to first declare everyone who opposes you as non-Muslim, then ask for proof from a Muslim denomination having those beliefs.

The other problem with your argument is if you allow Mullahs to pass verdict on other’s faith, then you have to live with the fact that all the sects of Islam have declared each other as non-Muslim at one point or the other… Shias, Sunnis, Deobandis, Barelvis, etc etc., have all been declared non-Muslim by others. If you are going to believe all such fatwas, then there’s no one left who’s still a Muslim. What makes you so sure you yourself are a Muslim? Whichever sect or school of thought you belong to, you have been declared non-Muslim. So if you follow your own logic, then you yourself are a non-Muslim, and hence have no right to pass judgement whether anyone is a Muslim or not.