There were more than one apostate bishop, starting with Marcion, Tertullian, and so on. There were also many an apostate Bishops of Constantinople, including Lucaris. Judas was an Apostle, yet Christ referred to him indirectly as the devil. Perhaps these people are there to test the faith, to challenge the Church. The Church is made up of sinners, FK, unless they are Reformed and God won't let them is no more.
BTW a Patriarch is not a "rank" but honor.
At the time, were his followers wrong to follow him?
Yes, of course they were. Obedience to a bishop is a given. But only if the bishop exercise economy (latitude within the doctrine), not if it is heretical.
What should his followers have tested his teachings against, if anything?
The whole wisdom of the Church: the scriptures, the patristic writings, the earliest documents, the Ecumenical Councils, hesychastic fathers...etc.
The Bible?
Have you ever read Philokalia volumes? They are laced with biblical references. There is no Church document that is not referenced to a Bible.
Cyril would get an "A" there
"A" for an apostate, yes.
How about the Eastern Church? Now here he would have failed of course, as proved out later
Not just the Eastern side of the Church, the whole Catholic and Apostolic Church.
We don't believe that God prevents us from sinning at all after salvation. Where do you get that from?