I will only address what I see is...curious?
I'm kind of surprised by this paragraph to be honest. I didn't know that Orthodoxy believed that it is in our nature to be sinful. This is surprising to me in all honesty.
Anyway, here is my understanding of it. Human beings are not naturally prone to sin. That would mean that God created us to be sinners. This would be a strange idea. Even after the fall of man, one cannot say that God changed the nature of man to become sinners. This would be a strange idea as well. The understanding of the fallen nature of man, is that man has been weakened in his nature or maybe a better word would be damaged or diseased? Another way to look at it, is that before the fall, man lived in the full grace of God, and thus he was righteous before God; the sin of Adam, turned humanity away from God, and thus rejected God's grace and humanity ceased to naturally be righteous. Anyway the point I'm trying to make here is that we don't see the nature of man to be sinful, and right now mankind lives in a state less than what they are suppose to be. In a sense mankind, along with the fallen angels, are the only creatures of God, that are not fulfilling our nature as we were made to be.
Well, I don't wish to go beyond what I know that we teach. This is not an area I've focused on in study (kept meaning to, as I am interested, but there is soooo much richness to explore).
So I want to be careful in what I say.
God did not create us to be sinners, no. Adam had the potential (theoretically) to have never sinned.
And no, we don't believe that God changed our nature.
However, we would say that human beings born after the fall ARE prone to sin. The world is infected by sin, sin multiplies, and because we have that bent, we will all sin if we live to be old enough to make choices and act on them.
(I actually know more about the Catholic explanation for this, as I listened to a program on EWTN one day, and wanted to come back to TAW and ask for our teaching on it, but got distracted.) So I don't think I will say more than that about the mechanics.
I need to do a little more study on this. But I know that Fr. Andrew Stephen Damick, and others I have listened to, have expressed this matter of Christ's nature as part of our concern.
The best way to look at this IMO and I don't think that it is a conundrum, is that between mother and child there is always a very unique bond of love, as having shared in the same flesh for 9 months. I don't see how Jesus would have been different, especially considering that Jesus is God, and God is Charity (Agape).
When we say that Mary is the Immaculate Conception we are truly saying that at no point, even at her conception, has the bond of love between her and her Son, ever broken. Is that not what sin is? Turning our backs on God's Love and Goodness, to love something else more?
I can see your point, but I am still uncomfortable with it. Something doesn't feel right to point to Mary's Conception and say that if it had been like the rest of us, it would represent a break of love between her and Christ. It almost even implies something like prexistent souls, since in order for love to be broken, it would to exist already. And it also seems to imply that the action of being conceived otherwise is an action taken by the child conceived, in order to result in a break of love. If it is something that happens TO us (being conceived) ... how can it be an action on our part breaking love? This begins to get philosophical, which can easily lead to error, but the whole thing feels not right. I suspect there has been more informed comment on the subject already than I am offering here. But what I do know just makes me uncomfortable with this.
I don't think that when one truly delves into the differences that there is truly an opposition. I know this would be a disagreement, but I do think that they are not irreconcilable.
For some of our disagreements this may be. There is quite often misunderstanding on both sides, and difficulty in truly communicating. If I didn't know Western thought before I started studying Eastern, I wouldn't likely be able to see how often that happens.
But there truly is an undercurrent of difference that colors this any many of our differences, stemming from the tendency of the East to have an overall view of sin and the fall as a sickness of our souls, that God desires to heal and restore, compared to the more legalistic tendency introduced by (Augustine, was it?) that Catholicism seems to emphasize. And I realize that we both include both of these aspects, but it is the emphasis that tends to separate us. However, in some cases, we may begin to understand each other better as a result of understanding what we do share.
But with all due respect (and I do mean that) ... I find a near-universal tendency for Catholics to play down our differences, but I don't know any Catholics who were once Orthodox, and so understand both ways of thinking. And it IS a different way of thinking that takes months or more likely years of immersion and study to understand.