It is strange that when it comes to a matter of consensus, you would oppose that with something that is the opinion of a particular group of people (Mawrid) who have come to the helm in recent times to take advantage of a people without access to Arabic.
You have taken in both cases an evidence used for the establishment of ijma', and specified an application whilst excluding the application you want it to exclude. When the hadith (s) said that my ummah will not unite on an error, you say that it does not apply to matters of ijtihad. There is no reason to insist that it excludes ijtihad on a matter relating to the religion, or pleasing Allah.
Furthermore there are numerous other evidences from the Quran and Sunnah.
Allah says "Inna nahnu nazzalna adh-dhikra wa inna lahu la haafidhoon." "Indeed we revealed the reminder and indeed we are preserving it."
This is not restricted to the Qur'an. The Dhikr/Zikr is anything to do with the religion that Allah has legislated for his remembrance. What is the point of Islam as the final revelation if the Qur'an but nothing else from the various opinions is preserved? Do you not see that if a scholar TODAY comes along and claim we can go against ijma' on any issue of religion no matter how small, we have claimed that Allah HAS NOT PRESERVED *THIS ASPECT* OF THE RELIGION until this modern-day scholar has come up with its "correct interpretation." So all of a sudden a 21st century scholar has corrected the error of the whole body of Muslim scholarship in issues of penal code/hudud? Not only is this contrary to common sense, but it is arrogance of the highest degree, and I would ask anyone who follows this organisation to check their hearts and ask Allah to guide them to what He knows is the truth.
Another evidence are the numerous ahadith detailing that there will never cease to be a group of people in the ummah that are steadfast upon the truth. If we go against ijma', we are again saying that Allah has left the Muslims to carry on for the last 1400 years with no one who was steadfast on the truth.