You asked about Mary being sinless.
The best way to think about this is to realize that she bore Jesus Christ, who is both God and man, in her womb. Mary is like the Ark of the Covenant and she had to be a specially prepared and holy vessel to carry God in her womb. In the Book of Revelation it says that nothing unclean will enter heaven. In the OT we read about Isaiah and his vision in heaven. Isaiah immediately became fearful because he knew he was a man of unclean lips. To be made clean an angel went to the altar and brought a burning coal to Isaiah’s lips and thus he was made clean. Moses had to remove his sandals when he walked upon the holy ground before God in the burning bush. I could go on but you begin to see the reason why it is appropriate for Mary to be completely holy.
Now back to Mary and the Ark. In the book of Revelation at the end of Chapter 11 John sees a vision of the Ark of the Covenant in heaven. Immediately thereafter John sees a great portent, “a woman clothed with sun.” This vision is of the Blessed Virgin Mary in heaven as is evidenced by the context that follows that harkens back to the book of Genesis where we see Satan, the Woman, and Her Seed[Jesus the Savior] who will rule with the rod of iron.
Now compare the following verses to see the relationship of Mary the Ark of the New Covenant with the Ark of the Old Testament. And remember how holy the Ark of the OT was. Uzzah, in Samuel 6:6, was killed instantly when he put his hand to the Ark when the oxen stumble. He was struck dead by God for simply touching the Ark. Now check out each pair of the following scriptuures.
2 Sam 6:2
And David arose and went with all the people who were with him from Baale-judah, to bring up from there the ark of God
Luke 1: 39
In those days Mary arose and went with haste into the hill country, to a city of Judah
2 Sam 6:9
And David…. said, “How can the ark of the Lord come to me?”
Luke 1: 43.
(and Elizabeth said to Mary) “And why is this granted me, that the mother of my Lord should come to me?”
2 Sam 6:10
David took it [the Ark] aside to the house of Obededom the Gittite.
Luke 1: 40. and she[Mary]entered the house of Zechariah
2 Sam 6:15
So David and all the house of Israel brought up the ark of the Lord with shouting
and she [Elizabeth] exclaimed with a loud cry,
2 Sam 6: 16
As the ark of the Lord came into the city…Michal saw King David leaping and dancing before the Lord;
Luke 1: 41
And when Elizabeth heard the greeting of Mary, the babe leaped in her womb;
2 Sam 6:11.
And the ark of the Lord remained in the house of Obebedom the Gittite three months;
And Mary remained with her [Elizabeth] about three months
So David went and brought up the ark of God from the house of Obed-edom to the city of David with rejoicing;
And Mary said, "…… and my spirit rejoices in God my Savior
These parallels are not mere coincidents in scripture. Think about it, the Ark of the Covenant contained the stone tablets of the law, Aaron’s rod(sign of the priesthood), and the manna. Mary, the Ark of the New Covenant, carries in her womb Jesus who is the fulfillment of the law and the prophets, our eternal high priest, and the bread that has come down from heaven.
In the book of Exodus we read about how God gave careful and extensive instructions to Moses on building the Ark. So also Mary was specially prepared for carrying Jesus in her womb. The Ark of the Covenant had the mercy seat from which God spoke to the high priest of the people. Mary carries God incarnate in her womb. How much more special and precious in purpose is Mary than the Ark of the Covenant in the OT?
Read the scriptures on all of this and you will see the reason Mary was “saved” in a special way by Jesus from the stain of original sin and was sinless throughout her life. It is all because of who Jesus is, and it is all by the love and power of God.