One note that is very relevant. Ijma' is a source of law. What is the source for Ijma' being an evidence? If it were Ijma' itself then that would lead to circular reasoning. Therefore the evidence of Ijma' is from the Quran and Sunnah itself. The other sects reject Ijma' as a source of law for obvious reasons. The likes of Jubbai are mentioned in the books of Usul. So it is irrelevant if the opinion of a mujtahid mubtadi' is taken in to consideration or not, simply because the innovators themselves do not believe in Ijma' as a source of law. Another point to note is that during the civil strife in the reign of Mawla Ali radiyAllahu anhu, despite the civil war, Mawla Ali radiyAllahu anhu was Khalifah. From what source do we establish this? By way of Ijma', even if there were dissenters.