Forgive me, but according to my understanding of Roman Catholic Christology and the EO-RC Joint Theological Declarations, I believe it would be an error to assert your church believes that the Blessed Virgin Mary is of a different nature, substance or essence from other humans, since Ephesus and Chalcedon declare our Lord is in a hypostatic union of two natures, His uncreated divine nature and the human nature He assumed in the incarnation, which are united hypostatically without change, confusion, separation or division. If the Theotokos actually has a different nature, which every Roman Catholic theologian I’ve met denies was the intention of the Immaculate Conception, then our Lord would not share our human nature.
Indeed a primary reason why the Orthodox object to the doctrine of the Immaculate Conception is because of concerns it would lead people to deny the full humanity of Christ our True God and His consubstantiality with us, that is to say that God became one of us, a man, in order to redeem our fallen nature, restoring and glorifying it through His Passion and Resurrection.
Jesus Christ is fully God and fully man without change, confusion, separation or division. Indeed St. Athanasius, who your church very strongly venerates, said God became man so that man could become god, becoming by grace what Christ is by nature.
I can indeed quote numerous Catholic liturgical texts that prove Christ is of the same essence as the rest of humanity.
Rather, the doctrine of the Immaculate Conception, if understood correctly, means that Theotokos was conceived without original sin, but with our human nature, but in objecting to the saying that she had the same nature as the rest of us, and by implication objecting, by accident, to the full humanity of her son, you illustrated the main concern the Orthodox have about the idea of the Immaculate Conception.
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I would also note that most concerns you might have about the Orthodox rejection of the doctrine are likely predicated on the basis of St. Augustine’s model of original sin, but we use the model of St. John Cassian’s Ancestral Sin, in which original sin is less about inherited forensic guilt and more of a hereditary disease that predisposes us to the sinful passions and causes us to be mortal. This model does an equally good job at precluding Pelagianism since no one afflicted with this disease could save themselves. However one can, through faith in Christ, who defeated death on on the Cross and remade humanity in his image, becoming the firstfruits of the resurrection, we can be saved, as the Blessed Virgin Mary was, and so close to God was she, closer physically than any other human by virtue of carrying Him in her immaculate and voluntarily sinless womb, that in response to her faith, which she affirmed in agreeing to carry Christ our God at the Annunciation, that when she reposed, she was taken up into Heaven bodily, which the Orthodox church has always celebrated, since the first century, on the Feast of the Dormition, celebrated by the Armenians at the end of July and by everyone else on August 15th, called the Assumption by the Oriental Orthodox, but which your church apparently did not formally dogmatize until Pope Pius XII declared it ex cathedra in the 1950s, which has the unfortunate effect of causing some anti-Orthodox polemicists to falsely accuse us of having followed the Roman church in adopting a recent doctrinal innovation (ignoring the fact that the Eastern Catholic churches had celebrated the feast since antiquity, and the Roman church historically was liturgically minimalist, particularly in August, probably due to the notorious heat in Rome in that month, and thus the Transfiguration was also never a prominent part of the Roman calendar).
At any rate the assumption of the Theotokos at the time of he repose is one of many indicators of her obedience, this being an honor previously granted only to a few, notably St. Moses and St. Elias, who as prophesied were present at the Transfiguration on Mount Tabor. Orthodox hymns describe her as Immaculate. But we do not say, nor do we need to say, that there was anything supernatural about her conception, because our hamartiology, while perfectly refuting Pelagius, does not require this. It should also be noted that historically the hamartiology of St. John Cassian was preferred in the West as well, until St. Augustine overtook him and all other Church Fathers during the early Scholastic period.
