Phred,
Regarding the sinlessness issue, could this be an area where the Roman and Eastern churches use different terminology and reasoning to describe essentially the same belief? Is there really that much of a difference between the "immaculate conception" and the contention that Mary lived an absolutely sinless life? My instinctive reaction is to ask for the Scriptural citation for the complete sinlessness of Mary, but I know that is a Protestant criteria that doesn't carry much weight in Orthodoxy. Not to sound irreverent, but how could anyone know? Not that I would dare to compare myself to Mary (or any other saint), but I know a lot of my own sin is between my ears....
Well, to answer your last question first, the Orthodox take literally (not figuratively) the claim made by Paul in the Scriptures that Christ is the head of the Church, and that we are the body. Indeed, that we have the "mind of Christ" in a very real (though not woodenly literal) way. It is fitting that Paul used a husband and wife to compare Christ and the Church: for in both cases two truly do become one in a very real way. This is why the Orthodox look to Tradition so much: we see in it (collectively) a manifestation and articulation of the will of God. We see the Church as a "living Scripture," of sorts. This is why, for example, that some have called the lives of the saints "applied dogmatics". So, without getting into a big post about epistemology, suffice to say that in Orthodox thought we accept that God reveals his will throughout history and not just in the Scriptures. And so, if God chose to reveal that Mary was sinless, then we accept this.
I guess it's possible that the Orthodox and Catholics are closer on the issue than I'm saying. Maybe I'm just defensive or used to taking a hard stance over and against what the Catholics teach. What I can tell you though is that, after reading books like Deification in Christ* by Panayiotis Nellas, and after having discussed these types of issues with Catholics in a friendly environment, there does still seem to be a wide difference. I think of it as seeing two mountains. Seen from far away they might seem very similar. Yet, upon getting closer you might notice that details are very different: and because of the difference in details, the whole character of each mountain differs. Standing on the Orthodox mountain, I don't exactly have a perfect view of the Catholic one... but from what I've seen, it does indeed seem different to me.
Regarding the sinless of Mary, it's important to understand that the word is used differently for her as compared to how it is used when speaking of Jesus Christ. When we speak of Christ being sinless, we mean that he is by nature sinless, that he is perfect and not in need of someone else's help. When we speak of Mary being sinless, we mean that she was helped by God and lived a life devoted to him, not having erred during that life.
The Orthodox (and even the Scriptures) often use such words with multiple meanings. For example, in the Divine Liturgy, it is said "Holy things are for the holy," meaning that the Eucharist is for those who are Orthodox Christians and have prepared. Yet, a second later, we say: "One is holy, one is Lord: Jesus Christ". In the first instance, we use "holy" in a very different way than it is used in the second instance: hence the reason that we could simultaneously say that the holy (members of the Church) can partake of the Eucharist, and yet that only one is holy. When I hear words like "sinless" attributed to Mary, or when we are said to "magnify" her, it's sometimes helpful to keep this in mind.
I'm not sure when the sinlessness of Mary became a firmly held doctrine. I know a couple Fathers of the early Church did not hold to it (I believe St. John Chrysostom was one of them). Much of the beliefs about Mary seemed to have been contained in Oral Tradition for the first few hundred years, with only scattered references and paintings/icons to clue us into the fact that there was something more than what the authors of written works were saying. If I could speculate, I believe that perhaps God held off on some things (like icons, the veneration of Mary, etc.) for a while, until the time was right. This is not to say that these things weren't around from the beginning: rather, it is known that the early Church was very secretive about some things, I just count these as some of them.
- Deification in Christ does not really go into the subject of Mary in depth, but it is the best book on anthropology from an Orthodox perspective I've read, and so has helped me to understand Mary in that way.