For one, to the OP's question, Eugenius IV issued the bull Etsi non dubitemus, which says the pope has superiority over the councils. Question closed, at least for Roman Catholics. I have tried to find a translation or even latin version of it, but I can't.
So, going off of what I can find people say it says -- not much to go on.
For booboo -- this really shows the big difference in our concepts of Truth and the role of councils, bishops, and what you term the magisterium.
The councils didn't write doctrine or create our faith. Our faith existed, the Apostles delivered it, and the councils defended it. They didn't improve, correct, add, etc. The faith is not subject to a majority or a vote. It simply is. The Church relies on the grace and guidance of the Holy Spirit to defend her. This is shown by consensus, not dictate from an office (whether that office is that of the Popes, or councils, etc). The consensus of the faith doesn't include the filioque. None of the Fathers used that creed. None of them write of the holy spirit in that way.
Second,
Pavia Siena Basel Ferrara Florence was hardly ecumenical. It was not convened to defend against heresy, it was convened for political purposes to unite both internally (Huss wars) and externally (against Islam).
Not all of the churches were represented. Moldovia's bishop was deposed
for leaving to go to Florence and they elevated another Metropolitan. Isodore represented Russia as a political appointee (basically forced on the Russians), but left as a Russian bishop and returned as a Roman Cardinal. The union was immediately rejected by the Russian bishops. Oops.
Don't forget, it wasn't just the Russians and Greeks at Florence. The Oriental Orthodox, Ethiopians, Coptic Christians, Armenians, etc were there. None accepted union with Rome on the terms Pope Eugenius presented (i.e. Submission to absolute authority).
It wasn't a "real" council. The money for the eastern bishops for their living expenses came from Florence, to Rome -- and the Pope withheld it for months at a time. The Emperor refused to leave the council until an agreement had been made because he wanted military aid. He placed limits on what his bishops could and couldn't say or debate. The list of who among the eastern delegation was allowed to vote on certain issues was changed from time to time throughout the council. The western bishops and the eastern bishops (St Mark especially) debated but there was no ground gained. Then, when Patriarch Joseph II died (at Florence) the eastern bishops said they couldn't hold to it without an eastern synod, because they had no authorized representative for Constantinople. (Much like St Maximos records the papal legates telling the Eastern reps at another time). The Eastern church didn't accept it, so the union never existed.
Look at it this way. Old councils were called (both by emperors and Popes) but the Council's ruling was what was supreme. Pope Eugene's bull made Council's non-authoritative, because only the Pope's ruling mattered. No ecumenical council can possibly happen under this structure, because it won't be a council...just an advisory committee.
