I’m afraid you totally misunderstand the concept of Catholic Church teaching.
The doctrine was infallibly formulated in 1854, but it should not be thereby assumed that the Church “invented” it at that time. No, in fact the formulation Pope Pius IX gave was merely an echo of Pope Alexander’s official teaching given way back in 1661.
[quote=Pope Alexander VII, S. O. Ecclesiarum, 1661]Concerning the most Blessed Virgin Mary, Mother of God, ancient indeed is that devotion of the faithful based on the belief that her soul, in the first instant of its creation and in the first instant of the soul’s infusion into the body, was, by a special grace and privilege of God, in view of the merits of Jesus Christ,
her Son and the Redeemer of the human race, preserved free from all stain of original sin. And in this sense have the faithful ever solemnized and celebrated the Feast of the Conception.
He goes on in this encyclical to condemn all opposition to the doctrine, and defended it against the concerns over it. The “ancient devotion” in his quote was also mentioned in the 5th session of the Council of Trent (convened 1546), under the treatment of original sin. They said that “it is not [our] intention to include in this decree, where original sin is treated of, the blessed and immaculate Virgin Mary, the mother of God;”. They then refer us to the decisions of Pope Sixtus IV, who reigned from 1471 to 1484, and who (sometime during his reign) made the Feast of the Immaculate Conception official.
Note, now, that this was quite some time before the Reformation. The Immaculate Conception doctrine was not a part of the counter-Reformation (though this idea is prevalent today), but it was already being celebrated for quite some time before. Still, it goes back even further. From the 15th century, we venture back and find that in the late 7th century it was being (unofficially) celebrated in the East. In the fifth (AD 446), Bishop Proclus of Constantinople, in his first homily, claimed that Jesus “proceeded from her [Mary] contracting no stain” because “he formed her without any stain of her own”. (Consider that “without stain” is the definition of “immaculate”.) And back in the third century (AD 235), Hippolytus wrote of his belief while commenting on Psalms 23.
[quote=Hippolytus, Commentary on Psalms 23]And, moreover, the ark made of imperishable wood was the Saviour Himself. For by this was signified the imperishable and incorruptible tabernacle of [the Lord] Himself, which gendered no corruption of sin. For the sinner, indeed, makes this confession: “My wounds stank, and were corrupt, because of my foolishness.” But the Lord was without sin, made of imperishable wood, as regards His humanity; that is, of the virgin and the Holy Ghost inwardly, and outwardly of the word of God, like an ark overlaid with purest gold.
This has been taught by Catholics since at least AD 235, then, and I haven’t even gotten into the Scriptural warrant. Your statement that the first time it was taught was in the 19th century is way off, and totally misunderstands the distinctions between “teaching” (in this case, it was taught since the Apostolic age), “officially teaching” (which began in this case in the 15th century), and “infallibly formulating” (which in this case occurred in the 19th century).
The prevalent idea of “original sin” at the time of Pope Pius, the idea he was writing under, was that “original sin” is merely a lack of sanctifying grace. To prove Mary was born without original sin, all we would have to do is prove (a) that she had sanctifying grace when conceived, or (b) that her soul had a special, graced condition. (We receive our souls at our conceptions, and they cannot change.)
Well, in Luke 1:28 Mary was addressed as “full of grace”, and a title from God always indicates something about the person’s inner nature. In other words, Mary was full of grace by nature, i.e. from conception onwards.
In Luke 1:46-47, “Mary said, “My soul glorifies the Lord and my spirit rejoices in God my Savior.” Certainly no ordinary soul could do such, because the ordinary soul is plagued by sin (1 Pet. 2:11) and sin cannot glorify or rejoice in God (2 Cor. 6:14-16).
So we have proven both (a) and (b). Mary had a special, grace-filled nature in her soul, and therefore could not have been conceived lacking grace (which is original sin), because that would be contrary to her nature.