Having given this more thought, I agree now that this is a theological difference that goes much deeper than just the timing of The Blessed Virgin's state of sinlessness. The Catholic mariology views Mary primarily are a second Eve, and secondarily as a saint among saints. Since Eve was made by God without sin, so must Mary; that completes the symmetry. It also places Mary's intercession on a different and more cosmic plane than the intercession of saints: she intercedes to Christ as Eve interceded to Adam and becomes uniquely the co-redeemer just as Eve was uniquely the co-sinner.
It is also Catholic teaching that Mary did not suffer the ordinary pain of childbirth, although I am not sure if it is dogmatic. Both are ancient beliefs East and West, even though the Immaculate conception was infallibly proclaimed very late.
So, what are we Catholics to make of the call to imitate her: "blessed are they who hear the word of God, and keep it" in Luke 11? I think it is on the same plain as the exhortation to become perfect in Matthew 5:48. Surely our Father in heaven had a different set of tools. So did Mary. It is not like there is a shortage of men and women equipped similarly as we are in the Communion of saints, priesthood and consecrated life, who become beacons of perfection for us.
Orthodox teaching is also that the Theotokos did not experience the ordinary pain of childbirth. As well as being the tradition handed down within the Church from the beginning, it is at the very least inconceivable to us that God the Son would inflict pain on his mother.
Given the fact that we believe that she was born with the effects of the ancestral sin, there is no reason why she could not have experienced pain (if one was willing to believe that God would not act in such a way as to spare her that pain), since pain in childbirth is part of the human condition. But the Orthodox tradition on this is very clear, so we don't need to rely on deductive reasoning.
I'm not sure that it has any dogmatic significance, but as with so many things, an Orthodox Christian will generally not see any reason to disbelieve traditional accounts.
We also see the Theotokos as a second Eve. We also see a symmetry. The first Adam had only to keep from eating the fruit. The second Adam had to go through the incredible condescension of becoming a man, suffering our lot with us, and then being crucified and dying. It was a lot bigger job to undo the sin of Adam than it would have been to avoid it in the first place.
What you seem to be describing is that God gave a "do-over" to mankind by having Mary born in the same pre-fall state as Eve was created in. Only this time he filled the second Eve with so much excess grace that it was pretty much a done deal that she would make the right decision. (Forgive me for painting with a broad brush -- I'll let you do the detail work!)
The Orthodox understanding is that this was not a "do-over" -- it was an undoing. And as with all undoings, this was extremely difficult. It took thousands of years to arrive at that point in time with that family, with that one young girl. And it was not a done deal (although of course the usual stipulations of God's foreknowledge, being outside time and history, still apply.)
If this was an act of God to create (so to speak) de novo a second Eve who would be free from sin and filled with grace from the beginning such that she wouldn't sin, then there is the question of why God would wait so long. The Orthodox understanding is that God didn't wait so long -- mankind *took* so long...
Likewise, the Theotokos accomplished what she did without the benefit of being free from corruption and the effects of the ancestral sin. Again, what the second Eve did was much greater than what the first Eve would have done had she refrained from disobeying God.
If anything, the Orthodox view of what the Theotokos accomplished is more elevated than the Catholic view, precisely because we believe that she had to overcome the tendency to sin and the corruption of body that comes with being born with the effects of the ancestral sin.
At least this is how I understand it.
It will be a major stumbling block. Agrarian has very aptly explained the Orthodox belief: the fact that she was subject to death does not make her a pre-fall Eve, even if she never sinned. You on the other hand show the Western Church's belief that for a co-sinner, there was a need for a co-redeemer. It sounds elegant but is not true by necessity: since Christ is the Second (pre-Fall) Adam, Mary must be the Second (pre-Fall) Eve.
I just see her as Theotokos, one human being who, if for no other reason than as a mother of the most precious Child, had to love God with all her mind, heart and soul and therefore "naturally" cleaved closer to Him than any of us can.