Further points on the Immaculate Conception are here:
http://www.freerepublic.com/focus/f-religion/1222429/posts
But to answer your point:
"How can you hold that Mary never sinned in light of Paul's statement...
....
Specifically, "for all have sinned and fall short of the glory of God"."
This statement must be capable of admitting exceptions, because we see his disciple/secretary St. Luke asserting the following:
Lk 1,5 "In the days of Herod, King of Judea, there was a priest named Zechariah of the priestly division of Abijah; his wife was from the daughters of Aaron, and her name was Elizabeth. 6. Both were RIGHTEOUS IN THE EYES OF GOD, OBSERVING ALL THE COMMANDMENTS AND ORDINANCES OF THE LORD BLAMELESSLY."
If you want a precedent of a Saint who was sanctified from the womb, then St. Luke also provides us with this when St. Gabriel announces the birth of the Baptist:
Lk 1,15 "for he will be great in the sight of the Lord. He will drink neither wine nor strong drink. HE WILL BE FILLED WITH THE HOLY SPIRIT EVEN FROM HIS MOTHER'S WOMB, and he will turn many of the children of Israel to the Lord their God."
***This statement must be capable of admitting exceptions, because we see his disciple/secretary St. Luke asserting the following: Both were RIGHTEOUS IN THE EYES OF GOD, OBSERVING ALL THE COMMANDMENTS AND ORDINANCES OF THE LORD BLAMELESSLY." ***
But it is clear from the context that Luke doesn't mean they were "sinless" and therefore exempt from the passage in Romans because immediately he recounts the story of Zechariah's unbelief...
"And Zechariah said to the angel, "How shall I know this? For I am an old man, and my wife is advanced in years." And the angel answered him, "I am Gabriel, who stands in the presence of God, and I was sent to speak to you and to bring you this good news. And behold, you will be silent and unable to speak until the day that these things take place, because you did not believe my words, which will be fulfilled in their time."
Luke 1:18-20
Zechariah was clearly punished because he "did not believe". He did something wrong and got rebuked for it by the angel. Lack of faith is a sin as Paul says here...
Romans 14:22
... For whatever does not proceed from faith is sin.
***If you want a precedent of a Saint who was sanctified from the womb...HE WILL BE FILLED WITH THE HOLY SPIRIT EVEN FROM HIS MOTHER'S WOMB,***
Truly he was filled with the Holy Spirit from the womb, and Jesus calls him the greatest of all those born of woman, but this didn't render him "sinless". If John were "sinless" then why would he appeal to Jesus...
"Then Jesus came from Galilee to the Jordan to John, to be baptized by him. John would have prevented him, saying, "I need to be baptized by you, and do you come to me?""
Matt - 3:13-15
He recognized that he need to be baptized by Jesus.