What is the Catholic response to the Eastern Orthodox view that the Immaculate Conception remove Jesus' ability to redeem all of humanity? I am discussing and debating the various subtle doctrinal differences between Roman Catholicism and Eastern Orthodoxy with a very close friend of mine. One question I've asked is, "How do Orthodox Christians view the Immaculate Conception."

His exact answer was:


  Orthodoxy believes the Immaculate Conception to be superfluous. Saint
  Gregory the Theologian (4th c.) writes regarding the Incarnation,
  "That  which has not been assumed has not be saved." To make the
  Virgin Mary free of the taint of original sin pulls her out of the
  human chain, robbing Jesus of the Father's will for him to assume all
  of our fallen humanity in order to redeem and save it. (2 Cor. 5:21;
  Gal. 3:13a)


I think this is a pretty solid argument. 

Especially the part that says:


  To make the Virgin Mary free of the taint of original sin pulls her out of the
  human chain, robbing Jesus of the Father's will for him to assume all
  of our fallen humanity in order to redeem and save it.


What explanation does the Catholic Church put forth to refute the statement above?

I think my question above differs from this question because I'm not necessarily asking for a scriptural argument (although it would definitely be welcomed).

To be clear, I am only interested in answers that are aimed at defending the doctrine of the Immaculate Conception.
There seems to be a thin line of differences between Roman Catholicism and Eastern Orthodoxy on the doctrine of Immaculate conception and seems to be in relation to the interpretation of doctrine of Original sin as seen here.  I though find hardly any difference between these two views, after reading this article. Here this work however  indicate that Eastern  Orthodoxy did have same views as Roman Catholic Church on this issue.   

Notwithstanding these two views, the doctrine of Immaculate Conception still hold good because a Son who is Divine in nature cannot come in a womb of a woman who is defiled (whichever view one takes, whether it is the source of man's fallen nature or transmitted mortality) by the very nature as human beings.  

God at the instance of conception in St Anna’s womb, Blessed Virgin Mary was preserved free from all stain of Original Sin. This was done by a singular grace and privilege granted by Almighty God, in view of the merits (Divinity) of Jesus Christ, the Saviour of the human race “.  

In Luke 1:47, Mary proclaims:  


  "My spirit has rejoiced in God my Saviour."  


This is referred to as Mary's pre-redemption by Christ.  

Like all other descendants of Adam, she was subject to the necessity of contracting original sin (Whether it is the source of man's fallen nature or transmitted mortality). But by special intervention of God, undertaken at the instant she was conceived, she was preserved from the stain of original sin and its consequences. She was therefore redeemed by the grace of Christ, but in a special way ---by anticipation.  

The Immaculate Conception does not imply that Mary did not need redemption through her Most Holy Son, Jesus Christ.  

From the Catechism of the Catholic Church:  

492 The "splendor of an entirely unique holiness" by which Mary is "enriched from the first instant of her conception" comes wholly from Christ: she is "redeemed, in a more exalted fashion, by reason of the merits of her Son". The Father blessed Mary more than any other created person "in Christ with every spiritual blessing in the heavenly places" and chose her "in Christ before the foundation of the world, to be holy and blameless before him in love".