You answered your own objections.Ijma or Qiyas is only allowed if it's also in-line with Quran and Hadith. Ijma or Qiyas against Quran and Hadith means nothing in Islam.
Going by your logic, if all the scholars today agree on allowing LGBTQ - will you be willing to marry yourself with the same gender?
If it is not in line with the Quran/Hadith then it is not valid to begin with. The whole concept of Qiyas and Ijma is to remain within the bound and folds of Islam.
That will never happen because the Messenge of God said that the majority of Muslims will never agree on something wrong.
There is a Hadith about the end times which means that "a time will come when men will marry men and women will marry women."
This whole lgbtq thing is a fad and is part of a concoction the Western ruling class came up with to distract the masses from the fact that most of them are living in debt, struggling paycheck to paycheck and there are not many high paying jobs for them.
Another reason for this noise is to forever silence the Black people from demanding reperations for one of the worst crimes in recent memory: slavery!