When the Church defines a doctrine, she merely makes explicit and of faith what had been hitherto implicit. She gave, not a new truth, but simply made a matter clear by defining the doctrine to be part of the revelation brought to us by Christ. When the Church speaks infallibly, it is Christ Himself speaking through His mystical body.
Mary was not the highest person in existence since Adam. Christ was infinitely higher than Mary. By mere fact of being a creature, she is infinitely lowly before the Creator.
The fact that the Our Lady was preseved from the taint of Original Sin was really an anticipated baptism, that is, the redemption of Mary's soul by prevention of sin's contamination. She owed her preservation from sin to the anticipated merits of Christ. The Eternal Son of God would not enter this world through a defiled doorway. She was granted this singular privilege of Almighty God because she was destined to be the Mother of God.
The Immaculate Conception does not imply that Mary needed no Redemption. She needed it as much as you and I do. She was redeemed in advance, by way of prevention, in both body and soul, in the first instant of conception. We receive the fruits of redemption in our soul at Baptism. The whole human race needs redemption, but Mary was separtated from that sin-laden humanity as a result of the merits of Our Lord's Cross being offered to her at the moment of her conception. If we exempt her from the need of redemption, we would also have to exempt her from membership in humanity. The Immaculate Conception, therefore, in no way implies that she needed no redemption. She did! Mary is the first effect of redemption, in the sense that it was applied to her at the moment of her conception and to us in another and diminished fashion only after our birth.
There had to be some such creature as Mary -- otherwise God would have found no one in whom He could fittingly have taken His human origin. Had there been no Immaculate Conception, then Christ would have been said to be less beautiful, for He would have taken His Body from one who was not humanly perfect! There ought to be an infinite separation between God and sin, but there would not have been if there was not one Woman who could crush the serpant's head. (Gen 3:15)
On account of the sin of Adam, we, his descendants, come into the world deprived of sanctifying grace and inherit his punishment, as we would have inherited his gifts had he been obedient to God. This sin in us is called Original Sin. This sin is called original because it comes down to us through our origin, or descent, from Adam.
What about Romans 3:23, "all have sinned"? Fundamentalists, as a rule, think it means more than that everyone is subject to original sin. They think it means everyone commits actual sins. They conclude it means Mary must have sinned during her life, and that certainly would speak against an Immaculate Conception. But is the Fundamentalists' reasoning solid? No.
Think about a child below the age of reason. By definition he can't sin, since sinning requires the ability to reason and the ability to intend to sin. If the child dies before ever committing an actual sin, because he isn't mature enough to know what he is doing, what act of his brings him under their interpretation of Romans 3:23? None, of course.
This is indicated by Paul elsewhere in the epistle to the Romans when he speaks of the time when Jacob and Esau were unborn babies as a time when they "had done nothing either good or bad" (Rom. 9:11). Thus there is a time in people's lives before they have sinned, meaning Paul's statement earlier in Romans must be a general rather than an exceptionless principle.
We also know of another very prominent exception to the rule: Jesus (Heb. 4:15). So Paul's statement in Romans 3 must also include an exception for Jesus. But if it includes an exception for Jesus, the Second Adam, then it also includes an exception for Mary, the Second Eve.