Apostolic churches (both Roman and Orthodox) are the authentic Christianity.
Protestantism is just like a "made in China" Christianity.
St. Mary is the Mediatrix in a secondary way. She's the channel which God came to this world (Mother of God) and all graces from God are distributed through (not from) her.
St. Mary being the Mediatrix of all Graces was accepted even by Martin Luther, the father of Protestantism.
Using the JW language, St. Mary's role is the same as the governing body (regarding the distribution of Grace).