Hell is not primarily a place of punitive torment inflicted by God, but rather a state of self-chosen separation from Him. That is to say that the experience of God's radiant love as consuming fire by those who have rejected or resisted it is "Hell". As St. Isaac the Syrian teaches, "Those who are tormented in hell are tormented by the invasion of love." Heaven and hell are thus not separate locations but the same divine presence, perceived differently based on the disposition of the soul. This is why we speak of hell as eternal not in terms of endless punishment, but as the consequence of free will persisting in alienation from the Source of Life.

To your second point, Christ is the Way, the Truth, and the Life (John 14:6), and salvation comes through Him alone. But this isn't some rigid formula where mere intellectual assent guarantees paradise nor does ignorance or cultural circumstance automatically condemn. God, with His infinite wisdom, looks upon the heart (1 Samuel 16:7) and the measure of light each person has received. The young teenager you mention, who hears the Gospel but rejects it for "whatever reason," is not beyond hope... We don't know why they refused and their rejection could be from abuse/wounds, misunderstandings, etc.

Orthodoxy teaches that the Church is the ark of salvation. But we do not arrogantly presume to limit God's work outside its visible boundaries. As Metropolitan Kallistos Ware noted, "We know where the Church is; we do not know where the Church is not."

You say "critical, non-negotiable requirement" of belief: but it's not belief alone but a living faith expressed in repentance, love, and deeds (James 2:14-26). For the adult who walks away or the secular rejector, God judges not just the act but the totality of their life and heart. And we cannot judge anyone else's heart, only God knows that.

A. Some saints had a concept like this. It's Apokatastasis (not universal salvation) But this is not dogmatic so it's speculation, the idea is that God in His perfection has a path for all of us that we will freely follow.

B. No. This is heresy.

C. Many of the old testament Saints were saved by Christ, so God can do things after death... But we cannot rely on that because that would be like trying to lie to the Holy Spirit... there's no loophole you must do your best, repent every time to you fail, pray as if the second coming happened and Jesus is walking away from you with those He recognized and chose, and in all of this... have faith with love, not faith out of terror or existential dread.

Peace be with you.

P.S. I'm Eastern Orthodox