That's what I thought. If a bishop severs communion with another, how does it affect communion with all of the others?
The final authority in the church is a bishop; where a bishop is, the fullness of catholic and apostolic church resides.
I thought findings of counsels had final authority, though they are or were only called in response to heresies invading a portion of the body. Am I wrong?
There would a Synod (Council) of bishops called by the patriarch or metropolitan of that particular church. The synod would review the grievances and the bishops would vote.
If the accused were to be found guilty of heresy, he and all those who agree with him would be given an opportunity to recant before the bishop(s) faced excommunication.
If the issue was a noncanonical practice, the Synod would organize a study group to investigate the nature and the need for a canonical change, whether it violated any scriptural tradition or whether it was in some ways contrary to the life of the Church in the past, whether it was in agreement or not with the Ecumenical Councils, and if there was readiness of the laity to accept or reject, etc.
The commission would present its findings and the Synod would vote. Until such time the old canon would be in force.
If a change was voted, it would have to be in agreement with the Church Holy Tradition, the Councils, the Bible and the Liturgical life of the Church all along. Once voted, it would be accepted as canon and all those who followed that canon would be in communion with each other.
I hope this answers your question.
General (Ecumenical) Councils proclaim dogma. They are binding for the whole Church. You are absolutely right that the purpose of these pronouncements was emergence of heresy that threatened the very foundation of our faith (Trinity, Christlogy, Mariology).
The problem the Orthodox have with poast-Schism Catholic dogmas is that some (like the Immaculate Conception) were proclaimed by the Pope bypassing the Synod, or perhaps pro forma addressing it with an accomplish fact, and that it was proclaimed without a visible or imminent heresy present.