Dear Anthony,

Corroborate does not imply derivation.  The Church has long-standing oral traditions which transcend the written corpus of both Scripture and other ancient manuscripts.  

A good example of this are the doctrines surrounding the Trinity and Christology.  While the Scriptures are in alignment with the teaching of the Church, nowhere in the Scriptures alone are all the facets of our understanding explained or clarified. That process of clarification and codification took place over centuries and often over rigorous debate.

In the same way, most Marian doctrines were posited in the context of Christological debate, and they helped to clarify what we know about Jesus.  

Even the Assumption, which wasn't defined until the last century, is a long-held belief that can be be understood in an implicitly forensic context: in a Christian history rife with turf wars over claims to saints; no one has claims to the body of the Virgin.  Furthermore, such a claim would be met with as much derision as someone claiming the remains of Christ. That claim never happened because the body was gone. Witnesses explained the events and history followed.

The Christian faith follows from real historical events met by real people, that has been transmitted through history.  Our Scriptures, however, are not simply a history book but rather a set of theological manuscripts that create narratives in such a way as to highlight particular truths that reveal the mystery of the Incarnate Word.  They are witnesses not just of history, but of the deeper truths that come from faith.  And still we know they are far from exhaustive (cf. John 21:25).  So, everything the Church teaches cannot be reduced to what is accounted for in any one source.  

So while the Church teaching may align with some of the Protoevangelium of James, the manuscript has never been canonized or endorsed completely.  Still, that does not preclude some of it being accurate.

Peace,