How would the rise of the Cathari lend support to that crazy notion? Almost every possible negative description of Catharist doctrine was given by their critics - how could they have omitted such a glaringly heretical claim?
Can't speak to "omissions," but can say that regional practice in that Cathar region during the 1st millenium established a special place for Mary Magdeline in local worship. And at least one local ruler believed himself to be "in the line of David." If memory serves, it was that regional leader who led the first Crusade to the Holy Land and became, for a while and with the support of the Templars, the "King of Jerusalem."