Sorry I don't live on the internet, but I do end up responding to things.
Mary is not listed in the Bible as anything other than a blessed, NORMAL woman. You have to twist enormous amounts of things to get anything else out of it. A clear and simple reading indicates she was a normal woman and the mother of other children
"Isn't this the carpenter's son? Isn't his mother's name Mary, and aren't his brothers James, Joseph, Simon, and Judas? Aren't all his sister with us? Where then did this man get all these things?" Matthew 13:54-56
Do you want us to believe Joseph had two wives? Did they adopt the other kids?
By the way, ALL have sinned. It is just that not ALL, such as babies and the mentally retarded are held accountable spiritually. We see this in the OT picture in the sacrifices, that there was always one for unintentional or unknown sins. In Hebrews we learn that Christ was the ONE TIME sacrifice for ALL sin. This covers the babies, the retarded, etc. It is not that they are not sinners, for they are -- the human nature is a rebellious one in its born state now (note God's words in Genesis 8:21 -- speaking of man He says, "every inclination of his heart is evil from his youth." No exceptions made.
It is not a matter of sin, therefore, for ALL have sinned. It is a matter of accountability.
Jesus saved Mary from original sin? The original sin was Adam's. He and he alone was accountable for that. It is sin NATURE we are all born with, or the tendency towards rebellion and evil. This is universal in man and Mary was no exception. The Bible shows nowhere that Mary is any exception.
Was Mary the Ark of the New Covenant? That's an interesting way to look at her, because the actual Ark of the Covenant is in heaven (Revelation 11:19). I sincerely doubt that this verse refers to Mary....
As far as handing Mary over to John's care -- if Peter was supreme, why not hand her to Peter?
However, to answer your question about why John, there are a couple of possibilities. John is known as the disciple that Jesus loved. This then would be the primary reason to trust Mary to him. However, and this has to do with tradition and so it can be discounted if you like, but tradition has it that Mary did not have other children until Jesus was around ten or twelve years old. One indication of this in the Bible might be when they did not see that He was with them returning from the Temple when He was 12. If He had been their only child at that point, they probably would have known exactly where He was. However, if they were busy with a baby or two, they would have simply counted on Him being with the group.
If this is true at all, then when Jesus was crucified at around 30-35 years old, his next oldest brother, if indeed a brother had been born ahead of the others, would be20-25 years old, maximum and in no position to financially take care of their mother. John, on the other hand, was much older and much more stable than any young person in their early twenties.
Please also, finally, note Acts 1:13-14
"They assembled in the upper room where they were staying...[listing the eleven Apostles]...these all continued with one accord in prayer and supplication, with the women and Mary the mother of Jesus, and with his brothers."
Note that the Apostles are listed first. Mary is simply grouped with the women. Then there are those other men -- Jesus' brothers.