On the one level, I'd ask in return how we are supposed to imitate Christ in his earthly life, we not being God.
On another level, I'd point out that we can, from this point forward, choose to live a life as holy as Blessed Mary did for her whole life, if we would but will it. The grace of God is there for us to never again fall into sin. It is in this sense that Blessed Mary is a direct role model for us. St. Augustine stated: "It depends on you to be elect" (In Ps. cxx, n. 11, etc.); "Who are the elect? You, if you wish it" (In Ps. Lxxiii, n. 5).
The main difference between the Virgin and us is that we have misspent the previous portion of our lives, up until now, since I am not aware of any of us being reknowned as living saints, while she did not. But we can now live a life as holy as she did from our age to her death, if we would but will it, and then follow through on that will. An ancient Roman Collect reads:
"Direct, we beg you, O Lord, our actions by your holy inspirations, and carry them on by your gracious assistance, that every prayer and work of ours may begin always with you, and through you be happily ended."
If we would only try to actually put that prayer into effect in our life, we would lead a life in imitation of St. Mary. God's grace is at the ready for us to do so.
I'll try to answer your other points later.
Your points are well taken, Hermann, but there is a difference. Although we are "washed" at Baptism, our nature does not become that of our original parents before the Fall. So, we continue to live with the propenxity to sin, because our nature is fallen.
This is where our theologies diverge, I believe, the Orthodox teaching that our nature has changed with the Fall, the Latins that our nature has become "tainted" but remians the same. If the Blessed Mary was conceived without sin and full of Grace, than her nature would have been that of Eve before the fall, and that would make her something that we are not, and her sainthood something we cannot attain even if we will it.