Sinlessness of Mary?
It is true that according to Catholicism, Mary is believed to be immaculately conceived and did not commit any real personal sin, but these graces did not preserve Mary from being tempted to sin.
Jesus himself was tempted in the ways men are; then logic tells us that his Mother would have been equally tempted as men are.
For we have not a high priest, who can not have compassion on our infirmities: but one tempted in all things like as we are, without sin. - Hebrews 4:15
Mary's Immaculate Conception was not necessary for our salvation!
The Catholic Church believes that it was not absolutely or intrinsically necessary for the Blessed Virgin Mary to be sinless; only fitting or appropriate (God making the choice).
Thus, even a dogmatic belief in the Immaculate Conception does not in any way impinge upon the Two Natures of Christ. We believe the Immaculate Conception happened, and are required to believe this, but we don't believe that it had to happen exactly as it did (in all possible worlds, etc.), in order for God to be God.
All the Immaculate Conception did was make Mary as Eve was before the Fall (precisely why the Church fathers often called her the Second Eve or New Eve): unfallen and sinless; not subject to original sin. As Cardinal Newman remarked, "Why is it considered such an extraordinary thing that God chose to simply make one person—the Mother of God the Son, the Theotokos—the way that all of us would have been, but for the Fall?"
Fr. Ryan Erlenbush wrote on The New Theological Movement website (12-8-11):
And this is the great difference between Jesus and Mary: She required a special grace to preserve her from the stain of sin, but our Savior (by virtue of the Incarnation itself, and on account of his divinity) could not possibly have contracted the guilt or debt of original sin in any respect.
While our Blessed Lady, even though Immaculately Conceived, required a Redeemer (namely, her own Son, Christ Jesus), the Lord did not have any need of a Redeemer. No special grace was required (beyond that of the Incarnation itself) to keep the Christ Child from original sin – he could not possibly have contracted it.
In this sense, the dogma of the Immaculate Conception is seen to be fitting and true while (in itself) not absolutely necessary, but the sinlessness of the Lord Jesus is absolutely necessary and super-eminently fitting.
Was Mary's Immaculate Conception Absolutely Necessary?
St. Thomas Aquinas maintains that it was indeed possible for Our Lord Jesus Christ to have been born of sinful woman; Christ's glory would not have been diminished thereby. But, he states elsewhere, it would have been inappropriate:
God so prepares and endows those, whom He chooses for some particular office, that they are rendered capable of fulfilling it, according to 2 Cor. 3:6: "(Who) hath made us fit ministers of the New Testament." Now the Blessed Virgin was chosen by God to be His Mother. Therefore there can be no doubt that God, by His grace, made her worthy of that office. ... But she would not have been worthy to be the Mother of God, if she had ever sinned. First, because the honor of the parents reflects on the child, according to Prov. 17:6: "The glory of children are their fathers": and consequently, on the other hand, the Mother's shame would have reflected on her Son. Secondly, because of the singular affinity between her and Christ, who took flesh from her: and it is written (2 Cor. 6:15): "What concord hath Christ with Belial?" Thirdly, because of the singular manner in which the Son of God, who is the "Divine Wisdom" (1 Cor. 1:24) dwelt in her, not only in her soul but in her womb. And it is written (Wis. 1:4): "Wisdom will not enter into a malicious soul, nor dwell in a body subject to sins."
We must therefore confess simply that the Blessed Virgin committed no actual sin, neither mortal nor venial; so that what is written (Cant 4:7) is fulfilled: "Thou art all fair, O my love, and there is not a spot in thee," etc. - Was the dogma of the Immaculate Conception formulated in response to someone challenging Mary's ability to bear a perfect Son?
If God preserved Mary from all sin, He could have preserved Christ's body and soul likewise without the grace of Mary's Immaculate Conception.
Was it necessary for God to make Mary immaculate at her conception so that she could be Jesus' mother?
No. The Church only speaks of the Immaculate Conception as something that was "fitting," something that made Mary a "fit habitation" (i.e., suitable dwelling) for the Son of God, not something that was necessary.
And hence they [the Church Fathers] affirmed that the Blessed Virgin was, through grace, entirely free from every stain of sin, and from all corruption of body, soul and mind; that she was always united with God and joined to him by an eternal covenant; that she was never in darkness but always in light; and that, therefore, she was entirely a fit habitation for Christ, not because of the state of her body, but because of her original grace. . . .
For it was certainly not fitting that this vessel of election should be wounded by the common injuries, since she, differing so much from the others, had only nature in common with them, not sin. In fact, it was quite fitting that, as the Only-Begotten has a Father in heaven, whom the Seraphim extol as thrice holy, so he should have a Mother on earth who would never be without the splendor of holiness - (Ineffabilis Deus).
St. Thomas Aquinas in his Summa clearly states that it was Adam and not Eve that was responsible for the transition of original sin. Now if Mary, the Mother of Jesus is the New Eve and Jesus Christ is the New Adam, it is obvious that the Immaculate Conception was not necessary for our salvation for the Holy Spirit can operate in that dwelling place He so chooses to. It the Holy Spirit that made it possible for Christ to be free of original sin and not Mary, the New Eve.
The solution of this question is made clear by what has been said. For it has been stated that original sin is transmitted by the first parent in so far as he is the mover in the begetting of his children: wherefore it has been said that if anyone were begotten materially only, of human flesh, they would not contract original sin. Now it is evident that in the opinion of philosophers, the active principle of generation is from the father, while the mother provides the matter. Therefore original sin, is contracted, not from the mother, but from the father: so that, accordingly, if Eve, and not Adam, had sinned, their children would not contract original sin: whereas, if Adam, and not Eve, had sinned, they would contract it. - Whether if Eve, and not Adam, had sinned, their children would have contracted original sin?
Thus Mary, like all of mankind, was saved by Christ’s victory over death on the Cross, albeit through special graces received by the Mother of Jesus.
Mary’s own victory is none other than Christ’s Victory. No other person had the possibility of offering their son’s death to God for the salvation of the world as Mary did at the foot of the Cross.