Which is an assumption on Catholics part. There is nothing that shows the Apostles had any knowledge of today's Marian doctrines. To protestants there are enough apparent conflicts with the Bible not to accept Holy Traditions. You can choose to hold Holy Traditions equivalent to the Holy Bible and adjust your interpretations of the Bible to fit. Protestants choose to put the Holy Bible above Holy Traditions and discount traditions that don't seem to match up. We are never going to resolve these differences, and personally I believe time is better spent on other things. It is an interesting topic, but I don't see it as God's will that we get consumed over these differences. I appreciate your concern that you believe you must change Protestants, but I think your effort is misguided.
Why, there is sufficient scriptural evidence that St. Luke and St. John at least were firsthand familiar with the life of Our Lady; we know from scripture that she was with the other apostles as well at Pentecost. St. Justin Martyr (early 2c) offers the cornerstone of all orthodox mariology -- the symmetry between Mary and Eve -- which only works if Mary is completely free from sin. If the Holy Tradition did not supply that knowledge to St. Justin, he had no reason to invent it and risk someone proving him wrong.
We are never going to resolve these differences
Thousands of converts from Protestantism each Easter prove otherwise.