"I would say first that your question presupposes that Scripture is the only authentic source of revelation in Christianity, and that would be wrong from the Catholic point of view....
I understand that you likely aren't too impressed with an argument based on Tradition as authority. But the early Church was. You have to ask yourself by what authority was the principle of Sola Scriptura (itself not specified in Scripture)enacted at the expense of other sources of authority known by the Church from the beginning."
If we both accept the premise that Scripture is "God's Word," how then can the "tradition" of man be as reliable a source of truth?
Man (and papal edicts) and "tradition" have changed in the past and still do, whereas the the Word never changes.
"We Catholics... consider the concept of Mary being conceived without sin and preserved in grace to remain sinless throughout life to be a 'settled matter' going back to the early Church."
Having been raised a Catholic, I understand the doctrine.
While the term "full of grace" or preservation of "sinlessness" throughout her life is not provable, nor scripturally specific, Mary was indeed "blessed amongst women."
There are two types of tradition, one with a small "t," and one with a capital "T." The small "t" tradition treats generally to disciplinary or rubrical practices, and can change. An example is clerical celibacy. It is not mandated by Scripture, although it is recommended by it. As a result, the Eastern Rite Catholics allow for a married clergy while the Western Catholics do not. Theoretically, the requirement for celibacy in the West could be abrogated tomorrow (though I seriously doubt it!), because it is a mere "tradition." So are other things, such as certain fasting rules, for example. These, too, could be changed or abolished tomorrow. They are not doctrine. But the Church still has authority in even these matters, and it should be obeyed in them because to it alone has the power of the keys been entrusted.
However, some other things involve Tradition with a capital "T." These things DO involve doctrine, and cannot be changed. They are part of the original Deposit of Faith, which comes down to us from the Apostles and the Church of the apostolic era. The canon of Scripture is a case in point. The Church was entrusted by God with the writing, compilation, vetting and canonizing of the contents of Scripture over the course of an over 350 year period. We believe that the Holy Spirit, promised by Jesus to guide the Church, does as advertised. The Church is called the "pillar and bulwark of the truth" in 1 Timothy 2:15. In order for it to be accurately described that way, the Holy Spirit clearly must guide it, preserving it from error in teaching. Were such protection not granted to the Church, we wouldn't have the Bible as we know it today, since no authority could have otherwise settled on the canon. The canon was a settled matter by the early 5th Century NOT because it was self-editing, and its contents were inherently manifest, but because the authority of the Church, guided by God to properly discern its Tradition in the matter, was able to *authoritatively determine* the canon of Scripture.
The authority of the Church, through Tradition and the teaching office (magisterium) competent to interpret both Scripture and Tradition, predates New Testament Scripture and has, indeed, given life to it. Catholics deny that *everything* that Jesus taught directly or through the Apostles as worthy of belief is contained in Scripture alone. The Immaculate Conception, at issue here, is another case in point. Direct Scriptural "proof-texting" is not something that lends itself well to the issue. But, from the earliest days, the Church has maintained the Tradition that Mary was sinless every moment of her existence. This wasn't even particularly contested by anyone until the 1500's. Exact definitions and understanding of the nature of how this came to be were lacking until Pius IX defined the matter in 1854, but he was acting to define a doctrinal point that the Catholic Church had "always and everywhere" believed. Ecumenical Councils, in conjunction with papal authority, act in a similar way in defining things that are, at some point in time, contested. Many of these things, found nowhere directly in Scripture, are nevertheless believed even by Protestants by default- the nature of the Trinity and the hypostatic union of Christ are examples of this. These things, too, are products of Tradition handed down through time and codified and defined as the need arises.
The Church has, does and always will understand itself in the context of having this authority. No one really denied the concept until the 1500's. One has to ask: "by what authority" did those men feel justified to leave the Church, deny what everyone had previously accepted, and even sever the logical connection to the justification of the Scripture they retained as the sole authority to be acknowledged? 1500 years into the Christian era, they were far more into "innovation" than any Catholic might be in any possible misapprehension of Tradition and the Magisterium.
That Protestants do not understand or accept the role of Tradition in Church teaching may be understandable, but it is so because they have cut themselves off from the roots of the Faith for nearly 500 years. Naturally, their own developing traditions do not adhere to Tradition as any valid source of authority. Their collective memory has lost the concept. But that's not our problem as Catholics. To those who honestly inquire about this and other matters that divide the body of baptized believers we can only say: "He who is able to receive this, let him receive it" (Matthew 19:12), and to the rest, we pray that their bias can be overcome, or that, failing that, God will be merciful in not holding them to account for things they were "invincibly ignorant" about.
Catholics do not need to be defensive when some things the Church teaches might not be clear from Scripture. But Protestants need to justify how they can ignore it when such a stance only saw the light of day over 1400 years after the death of the last apostle, when the deposit of faith was sealed.