The question of why France became obsesses with Mary Magdalene is an interesting one, and one I’m not entirely sure I understand. Differences emerged when the Western Church, following Gregory the Great but particularly after Odo of Cluny in the tenth century, identified many of the various women of the New Testament—Mary of Bethany, Mary Magdalene, the “sinner”—as Mary Magdalene, while the Eastern Church held that they were different women. Thus, in the Greek East, Mary Magdalene was never a very important figure, simply one of many women. But in the Latin West, because all of these women were thought to be the same, she had a larger impact.
I know that the ninth century French monk Christian of Stavelot (Christianus Druthmarus) is an important figure in the development of the modern Magdalene myth since he wrote that she was an apostle and evangelist, thus feeding directly into Wolter’s belief that she was the apostle hiding behind the name St. John the Less. Unfortunately, while I know this from secondary sources, I am not familiar with Christian’s works to know where he said this. Medieval hagiography depicts the Magdalene as Christ’s apostle down to the Reformation, at which point the version of the story Wolter knows and opposes emerges. The supposed cover-up he senses is partially true in that the Church of the Reformation period did try to downgrade the Magdalene. But the same Church had also venerated her for centuries before that!
It is only after the tenth century, though, that the Magdalene legend really blooms, and part of that is due to the relic trade. We know from Sigebert of Gembloux, in the Chronicon sive Chronographia of 1111 or 1112 CE, that in 745 bones said to belong to Mary Magdalene were removed from Aix, where she supposedly died after decades of solitude (a story imported from the third century account of Mary of Egypt), to Vézelay. Whether this actually occurred is less important than the evidence it provides that multiple places in France claimed to hold the bones of the Magdalene, in direct opposition to the Eastern tradition that they had not left Ephesus until Emperor Leo VI removed them and took them to Constantinople in the late 800s. Leo’s translation of Mary is recorded by John Zonaras (Extracts of History 3.143) in the twelfth century and George Kedrenos in his Compendium of History in the eleventh century. Obviously, this causes a bit of a problem, since choosing one legend or the other demands that we declare the other wrong—but on what grounds? Frankly, there’s no good evidence to accept either of them as anything more than medieval fantasy.
But the fact that the French (or, more accurately, Provençal) tradition can’t be traced back before 1100 CE, while the Ephesus version goes back to last Antiquity or the Dark Ages, strongly implies that there is no truth to it. Worse, if Mary Magdalene’s preservation of the Holy Bloodline in France was so big a secret, why would the all-powerful Church have sponsored so many churches dedicated the Magdalene and allowed the publication of hagiographies telling everyone she moved to France when there was a perfectly acceptable and widely believed alternative that would have kept the secret much better hidden?