So, all of the 63 Orthodox priests and all of the Orthodox Christians who observed the Treaty of Ungvar (Uzhhorod) in 1646 to join the Ruthenian Byzantine Catholic Church are now suffering the flames of fire and eternal damnation in hell? Is that what you are saying here?
Only the Orthodox Church is the One Holy Catholic and Apostolic Church. It is apostasy to abandon it for anything - whether it is Roman Catholicism or Lutheranism or.... The apostasy will endanger the person's salvation
This is the teaching of our Saints and Holy Fathers.
Is someone who has never belonged to the Eastern Orthodox Church and belongs to one of the other groups that you mentioned (or other denominations) still in sin, and if so is it still apostasy or something else?
... the words of the holy Metropolitan Philaret who was the First Hierarch of the Russian Orthodox Church Abroad when I was a young man and a very conservative theologian. He is here speaking of the salvation of heterodox Christians but I would think he would say the same about Jews and others:
"It is self evident, however, that sincere Christians who are Roman
Catholics, or Lutherans, or members, of other non-Orthodox
confessions, cannot be termed renegades or heretics—i.e. those who
knowingly pervert the truth... They have been born and raised and are
living according to the creed which they have inherited, just as do
the majority of you who are Orthodox; in their lives there has not
been a moment of personal and conscious renunciation of Orthodoxy. The
Lord, "Who will have all men to be saved" (I Tim. 2:4) and "Who
enlightens every man born into the world" (Jn. 1.43), undoubtedly is
leading them also towards salvation In His own way."
N.B: "The Lord...undoubtedly is leading them also towards salvation
In His own way."
And we have the words of St. Theophan the Recluse to guide us into a correct Orthodox understanding:
"You ask, will the heterodox be saved... Why do you worry about them?
They have a Saviour Who desires the salvation of every human being.
He will take care of them. You and I should not be burdened with such
concern. Study yourself and your own sins... I will tell you one thing, however:
should you, being Orthodox and possessing the Truth in its fullness, betray
Orthodoxy, and enter a different faith, you will lose your soul forever."
I am sure there were mitigating circumstances which may rescue them from damnation but yes, it's a possibility.
I think that it is up to God to decide, and not St. Theophan the Recluse, who does and who does not go to hell. BTW, does St. Theophan the Recluse have a higher status in your Church than Jesus? I noticed that Jesus says in Matthew 25: "  Then shall the king say to them that shall be on his right hand: Come, ye blessed of my Father, possess you the kingdom prepared for you from the foundation of the world.  For I was hungry, and you gave me to eat; I was thirsty, and you gave me to drink; I was a stranger, and you took me in:
 Naked, and you covered me: sick, and you visited me: I was in prison, and you came to me.  Then shall the just answer him, saying: Lord, when did we see thee hungry, and fed thee; thirsty, and gave thee drink?  And when did we see thee a stranger, and took thee in? or naked, and covered thee?  Or when did we see thee sick or in prison, and came to thee?  And the king answering, shall say to them: Amen I say to you, as long as you did it to one of these my least brethren, you did it to me."
I don't see any indication here that if someone follows his conscience to the best of his ability and changes Churches based on a conscientious decison, that he will lose his soul forever?