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.
Why is this question posed anew by the doctrine of the Immaculate Conception? The economy of salvation is such that the Jewish culture had to mature, through repeated bondage and liberation, so that the standard of ritualistic purity at the core of Judaism could culminate in the Immaculate Conception that men would pray for, recognize (on some mystical level) when it happens, and protect from defilement, as the Protoevangelium shows.
Outside of tradition, I don't know why she couldn't have experienced birthing pains either. That was one of the few explicit consequences of ancestral sin, so if she had it, then why not? We also know for sure that Jesus felt pain, so why not Mary.
I don't understand why there is the view that if Mary had birthing pain, that it would be Jesus' fault or causing. Nobody has ever blamed the baby for that, the reason for it was already known to all who knew their scripture.
Plus, if we carry this idea forward, that Jesus could not have "caused" his mother any pain, then one has to say things like Mary's back never hurt after lugging Him around as a tyke. Or, that Mary never stubbed her toe while running after Him. Under this standard, we do know for sure that Jesus "caused" Mary considerable emotional and psychological pain, since it was within Jesus' power to not go to the cross (in His capacity as God). So in a sense, Jesus did inflict pain upon His mother.