Yes! We positively deny the existence of Original Sin, as do the Fathers.
"If the Catholic doctrine of the Immaculate Conception is reformulated in positive terms, as the assertion of Mary’s possession of the Holy Spirit from conception, does the doctrine then become acceptable to the East?"
No! The Church has not spoken definitively on when The Most Holy Theotokos became "κεχαριτωμενη". There is all sorts of speculation about when, some saying in the womb, some saying when she enetered the Temple as a child, some when the Archangel Gabriel appeared before her. Personally I believe it must have been at or before she reached the age of reason. +John Chrysostomos himself argued that Panagia was not in fact sinless, though admittedly this speculation was not within the consensus patrum. The fact of the matter is the Fathers always admitted that they were speculating about this.
"And if Catholics and Orthodox can agree on the original purity of the Theotokos, do they not in fact essentially agree on original sin?"
Absolutely not! Do you understand what "Original Sin" is? Isn't that covered in RCIA? Saying that Panagia was sinless from conception says nothing about the existence or non-existence of "Original Sin".
Marys Fiat “Let it be done to me”
By Mary saying yes...This reverses Eve,s Sin in the perfection of Mary
As St. Irenaeus says, “Being obedient she became the cause of salvation for herself and for the whole human race.” Hence not a few of the early Fathers gladly assert. . .: “The knot of Eve’s disobedience was untied by Mary’s obedience: what the virgin Eve bound through her disbelief, Mary loosened by her faith.” Comparing her with Eve, they call Mary “the Mother of the living” and frequently claim: “Death through Eve, life through Mary.”
The following is an excerpt from http://209.85.165.104/search?q=cache:gWwECLkKDUwJ:communio-icr.com/articles/PDF/RKereszty.pdf+Jesus,s+fiat&hl=en&ct=clnk&cd=7&gl=us
When Mary said yes ,she has implicitly yet wholeheartedly accepted in faith all that has been said to her by the Lord as she accepted the Word made flesh into her womb; moreover, this initial acceptance of the sacrifice of her Son becomes explicit at the foot of the cross. Her consent to the Father in that she allows the self-donation of her Son to the Father to take place in our stead and for our sake is full and wholehearted because she is the Immaculata, the one conceived without original sin and full of grace. Therefore her Yes, her acceptance, is not weakened or divided by any sinful tendency. In this way, Mary who became the Immaculata by being fully redeemed in advance by her Son’s sacrifice, makes the sacrifice of her Son fully her own. And in so far as Mary is the archetype and the beginning of the church, in her the sacrifice of Christ has become the Church’s sacrifice. The patristic phrase “personam ecclesiae gerens,” “in persona Ecclesiae,”denotes a kind of representation that is truly valid only when the role that is played (persona) potrays precisely the subjectivity of the Bride-Church. But how could a sinner be capable of playing this role, which demands spotless love. The disposition he would have to portray would necessarily always be something high above him, an ideal that had not been realized, so that it would not be possible anywhere for the Church to play the role assigned to her in Christ’s sacrifice in keepin with what was expected. This is why the dogma of the Immaculate Conception is a strict postulate of ecclesiology... The assent of the Ekklesia to the sacrifice of the Son must press on until it reaches Mary’s perfect selflessness, so that this agreement may not retain any stain of the egotism that allows Jesus the Paschal Lamb to be slain for one’s own redemption and perfection .
end of excerpt
Just as we see a prefiguring and “types” of Old Testament faithful men Abraham,Moses etc....Culminating to PERFECTION with Jesus, so do we see the prefiguring “types” of faithful women Sarah,Rebecca ,Miram etc.. culminating to PERFECTION with Mary.
This is all I can give you from the Catholic perspective tonight.
I have a busy week ahead as I said in a previous post.
I,ll try and address this further If I can find the time.
I wish you a Blessed evening!