The thing about conciliarity is that no one bishop is set apart above the rest. No one specific bishop makes or breaks an ecumenical council. Vatican II has been declared Ecumenical, despite opposition from canonically Catholic bishops (the SSPX). So obviously something has been declared Ecumenical without the full unity of all the bishops in the Catholic Church.
The thing about conciliarity is that no one bishop is set apart above the rest, like you mentioned, meaning that the following could never happen within Eastern Orthodoxy, regarding any one bishop of any one self-governing church - correct:
“Pope Benedict XVI has declared that, for doctrinal rather than disciplinary reasons, the SSPX has no canonical status in the Catholic Church and, because of that lack of canonical status, the ministries exercised by its ministers are not legitimate in the Church.”
So, if this happened within Eastern Orthodoxy, regarding one or more autocephalous churches, as opposed to the SSPX, then how could an Ecumenical Council still be possible within Orthodoxy, since, as you claim, it’s not possible within Catholicism, without the full unity of all the bishops in the Catholic Church?
Perhaps ecclesial unity, via the Petrine office, or whatever one wants to call it, when conciliarity discord occurs, is a viable solution? Maybe, just maybe that was why Jesus said: “And I tell you that you are Peter, and on this rock I will build my church, and the gates of Hades will not overcome it.”
If you are right then what other way would there be for the CC to once again, declare something ecumenically, since, as you put it, the full unity of all the bishops in the Catholic Church has been lost?