That which mankind attributes God outside what He disclosed to us in the Bible (which has never included the Apocrypha) is the fancy of men.
Those ideas of mankind which elevate him (or her) above what God has declared about us in the Bible are mere fancy of men.
Mary was sinful as testified by the Scripture that all are sinners, none do good, all have sinned and fallen short of the glory of God.
There is NOTHING in Scripture that tells us Mary was born without sin. That's the fancy of men. As you say, "Catholics believe" it. That's mere fancy. Some in your church have taken Mary worship to cultic extremes, as evidenced by all the talk about her being the co-redemdtrix (sic?) with Christ. That's beyond fancy and is clear-cut heresy.
Religion of man - false traditions of men. Do not hold dearly to these as they will lead you away from the Lord Jesus.
Have you noticed that the ONLY sites which purport to be Marian apparitions that have been 'approved' by the Church are those at which Jesus IS glorified, and are ones where people are led TO Christ? There are a lot of kooks in the world, and one can claim any sort of 'apparition', but the Church is very wary about putting any store in the claims before some diligent research. If Mary is helping to lead God's people, she would be leading them TO Him, not away from Him. As for the title of Co-Redemptrix; it is simply a statement of the fact that Mary was an active participant in the Redemption of man by virtue of her "Yes" to God's invitation to be that "Woman of the Promise", foretold in prophesy. It was she who brought forth Jesus into this world, as the Father had planned, so she is an important figure in our Redemption. The Church does not teach that she did the redeeming, only that she was the one who God chose to make it possible for us.
Catholics venerate and honor Mary, as led by Jesus's own example; we do not worship her. As for the 'tradition' not being explicitly in the Bible, and thus you find it difficult to accept, what about the teachings of Jesus that ARE explicit in the Word of God that you don't accept, such as the primacy of Peter, and the Real Presence?
We could bandy Scripture about all day long, but as far as tradition is concerned, I'll take the word of folks who were with Mary while she was alive, and who passed her story along to those Church fathers who DID keep her story alive for the successive generations. They didn't make it up out of whole cloth a millenia later; they continued her story from its inception. As for the idea that Mary couldn't possibly have been conceived without sin, I personally believe that anything is possible with God, and if He wanted the mother of His Son to be sinless, so as to be a worthy vessel, He certainly could have done so. Far be it from me to put any limits on what He can do. And as was pointed out before, Christ was fully human, as well as fully Divine, yet he was also born without sin.