Mark Bonocore provides some very lucid and penetrating insights into the Church's teaching about Mary's sinlessness.  The questioner's comments are in brown.  Mark responds in standard text format (black).
od could not provide Jesus with a singular, unfallen human nature and STILL have Jesus have some connection to the rest of us.  However, by applying the merits of Christ to His mother "outside of time" (and so Baptizing her into Christ in advance), Jesus was able to both come from the line of Adam (through Mary) AND take on an undamaged humanity, like that of Adam (and Eve) before the Fall.  And, indeed, this was clearly God's plan from the start, per Genesis 3:15, which places "the woman" (a New Eve) in opposition to the serpent, bringing the Messiah out of that – the Seed of the woman pitted against the seed of the serpent.  Otherwise, Jesus would not have taken on a sinless and uncorrupted human nature, but our fallen nature, and so He could not have been the New Adam, but a fallen son of Adam like all the rest of us.  In this, one must also keep in mind that the sin which plunged humanity into darkness was that of Adam, not of Eve.  Eve did sin first; but until Adam, as head of the human family, joined her in sin (instead of remaining faithful and interceding for her), humanity as a whole was not cut off from God.  This is the problem which the New Adam came to repair.  And, since woman was not the cause of mankind's final break with God, God takes the creation of woman and re-builds the human race from her (a New Eve – Mary), using her to bring the New Adam (the God-man) into the world, which, in the very same dynamic, makes this New Eve's sinlessness possible.  As for John the Baptist, he was born (NOT conceived) without Original Sin so that he would have the authority to Baptize and to preach repentance for Judaism in expectation of the Messiah's appearance.  If all of Judaism has accepted his Baptism of Repentance (the ultimate mikveh – a return to fidelity to the Law), then they would have been prepared to receive the Messiah in holiness, and the Cross may not have been needed.  God, however, knew that it would be necessary. But, the open invitation was there, and it was very real.  Thus, John the Baptist was the real deal; that is, he was perfectly suited to preach holiness and perfection to the Jews, since he himself was holy and perfect (a sinless human being like Elijiah before him).
Rather, and perhaps tangentially, I see a more plausible argument in favor of necessity when it comes to examining the totality of her role in salvation history: Most especially as seen in Genesis 3:15 as the "woman" who is at opposition and "enmity" with Satan.  That, to me, demonstrates a necessity for purity and fullness of grace (as demonstrated and confirmed by Gabriel at the Anunciation) – more so than an argument about Christ's inheritance of human nature devoid of the effects of Adam's sin.  What am I missing here?  There are, no doubt, a few more things you can teach me.
Nope. :-)  I think you got it.  The only thing is that we must connect Genesis 3:15 to WHY Genesis 3:15 was necessary, as described above.
Also, does one's understanding of original sin come into play here too?  If one were to look at original sin as an actual sin affecting one's nature (since many Protestants blur the distinction between actual and original sin) that must, in the case of Mary or Jesus, be "filtered out" by the Holy Spirit at conception – as contradistinct from the notion that original sin is really the lack of the fullness of grace (as something good that is missing instead of viewing it as something bad that is attached).  Would that perspective impact how one would perceive the "necessity" or the "fittingness" of the IC?
Sure.  Protestantism, being rooted in nominalism as it is, invariably confuses Original Sin with "Original Guilt".  But, what Original Sin really is is a "macula" – a "dark spot" – a "place void of light" – that is, the light of God's grace.  And, along with this lack of grace is a mysterious "knowledge" ("intimate experience") of sin – the very thing that makes us weak to sin and inclined to commit it.  Jesus and Mary, however (like Adam and Eve before the Fall), had no "knowledge" of sin whatsover.  John the Baptist would have had some (because he was not conceived without sin).  However, the only way a totally sinless human like Mary could have sinned was (like Adam and Eve) if she freely and intentionally chose to offend God – a direct insult and totally willing disobedience.  Mary could not, for example, claim to be tempted out of weakness or appetite, since these things (unlike with us) were simply not in her.  And, as for Jesus, He could not have sinned even if He wanted to, because, as a Divine Person, He could not sin – even though His human nature felt the temptation.  However, a nature cannot sin.  Only a person can sin.  And, since Christ is ONLY a Divine Person, to sin He would be offending Himself which is not a possibility. :-)
Additionally, you state: "and that humanity had to be prepared in advance, or else the Incarnation (as Christians understand it) could not have taken place."  Why "in advance" necessarily?  If the "sinful" nature of Christ had been "filtered out" – or "filled in" with the fullness of grace (depending on how one views original sin) – by the Holy Spirit at His conception (just as we claim that Mary's humanity was at her conception) then how does this affect the Incarnation as Christians understand it?
What you propose above is very similiar to what some Greek Orthodox say when they seek to downplay or deny the Immaculate Conception (which is only defined in the Catholic Church).  In other words, operating on the Greek theological principal of Deification (their version of sanctification which stresses a sinner's "becoming like God"), the Orthodox (or, just some of them, I should say) maintain that Mary was made sinless when she gave her "yes" to God at the Annunciation, and that this "deified" (read: sanctified) both her humanity and Christ's own humanity in her womb.  However, the problems with this are many.  First of all, in Luke 1:28 (10 verses BEFORE she gives her "yes" in Luke 1:38) Mary is called "Kecharitomenae""Full of grace" or, more properly, "She who is perfectly graced."  This could not be possible unless Mary was already sinless prior to the angel's arrival – that is, before her "yes" to God.  Likewise, if the humanities of both Mary and Jesus Himself were "deified" ("sanctified") at the time of Christ's conception, then this means a) That Jesus drew His humanity from a fallen source and, even more problematically, b) that mankind, in the prototypes of Mary and Jesus, WAS NOT redeemed by the INCARNATE Son (the God-man), but by the Son INDEPENDENTLY of His Incarnation!!!  And, if this is the case, then the Incarnation would not be necessary at all.  Rather, the eternal Son could have remained in Heaven and "zapped" everyone on earth with sinlessness from there. :-)  However, that's obviously not what we believe.  We do not believe that Mary became sinless via the eternal Son's (spiritual) Presence entering her womb, nor do we believe that Jesus' own humanity became sinless because of the eternal Son's "taking on" of it.  Rather, we believe that the God-man (the Incarnate Christ – the New Adam) redeems humanity and we believe that this happened because 1) the saving merits of the God-man were applied to His intended mother outside of time / in advance (rendering her sinless), and that Jesus took His own sinless humanity from her – that is, Jesus also "redeemed" His own humanity in advance (in redeeming His mother's) by applying the merits of own His Incarnation to His mother, and then taking that sinless humanity from her.  Now, obviously, this is a great and profound mystery which must be contemplated prayerfully.  But, it is very consistent and air-tight, and in full accord with the Apostolic Deposit which is, of course, what the Church had to thoroughly examine (taking centuries to do so) before it dogmatized the Immaculate Conception.
Mark Bonocore
September 1, 2004