The Messiah had to come and be born of a woman. Mary isn't sinless, and never was. She was a virgin who honored God.
Romans 3:23 (KJV) For all have sinned, and come short of the glory of God;
That chapter in Luke is largely speaking of how all of those things came about, and Mary worshiping God and calling GOD holy. Have you ever noticed that Jesus is never recorded in the Bible as calling Mary "mother" until the cross? He was looking after her. She had no husband nor son to care for her, so He made sure John took her under his wing.
Mary is not EVER mentioned as a redeemer or source of our salvation. Mary is NEVER mentioned as being called God or the mother of God (as a title). The whole Bible points to Christ, NOT to Mary. Salvation is from God alone. (And btw, as far as Peter being "the stone" ...look Who is the stone! Jesus Christ of Nazareth!)
Acts 4:10-12 (KJV)
10 Be it known unto you all, and to all the people of Israel, that by the name of Jesus Christ of Nazareth, whom ye crucified, whom God raised from the dead, even by him doth this man stand here before you whole.
11 This is the stone which was set at nought of you builders, which is become the head of the corner.
12 Neither is there salvation in any other: for there is none other name under heaven given among men, whereby we must be saved.
Who is it that is full of grace? Not Mary!! lol
John 1:14 (KJV) And the Word was made flesh, and dwelt among us, (and we beheld his glory, the glory as of the only begotten of the Father,) full of grace and truth.
Righteousness and redemption comes through faith in Christ! Do we see see Mary named a co-redeemer? Absolutely NOT!
Romans 3:21-26 (KJV)
21 But now the righteousness of God without the law is manifested, being witnessed by the law and the prophets;
22 Even the righteousness of God which is by faith of Jesus Christ unto all and upon all them that believe: for there is no difference:
23 For all have sinned, and come short of the glory of God;
24 Being justified freely by his grace through the redemption that is in Christ Jesus:
25 Whom God hath set forth to be a propitiation through faith in his blood, to declare his righteousness for the remission of sins that are past, through the forbearance of God;
26 To declare, I say, at this time his righteousness: that he might be just, and the justifier of him which believeth in Jesus.