Actually Byzantium was in full communion with the Bishop of Rome when it fell to the Turks, having accepted reunion at the Council of Florence in 1439.

Stop saying this stupid crap. I see this all the time on this sub. You are just alienating Orthodox Christians more when you say stuff like this, which isn't true at best and downright offensive at worst.

First of all, Florence is repudiated almost immediately. Furthermore, it was a mostly Greek-attended Council, and the bishops who signed there mostly did so under duress. Additionally, the other three Patriarchs (Jerusalem, Antioch, and Alexandria) condemned it, so the only Patriarch who might have been a signatory was the Ecumenical Patriarch, who was conveniently found dead with a will saying the pope was right about everything during the middle of the council. The Russian Church declared itself autocephalous directly as a result of the council so that it wouldn't be influenced toward union by elements of the Byzantine Empire.

Orthodoxy isn't like the Latin Church-- a couple dozen bishops can't speak for everyone. Some Byzantine bishops signed, that's all. Saint Mark of Ephesus wasn't the only bishop who didn't sign-- he was the only bishop present at the council who didn't sign.