Hi. St. Dominic being asked to spread the Rosary is not just a “legend” - here is a Dominican web-page tackling the subject:
The Catholic.com article, mentioned by another poster, is quite full of research and good things, only to say, that Our Lady is Queen and Mother of Saints (the article does use this title lower down the page)…and, the obvious argument to this:
“Many non-Catholics object to the practice of asking the saints in heaven, including the Virgin Mary, to pray for us. Often cited is 1 Timothy 2:5, “For there is one God and one mediator between God and men, the man Christ Jesus.” Since Jesus is our only mediator, they argue, Mary (or any other saint) shouldn’t be asked to pray on our behalf.”
- is: Jesus is the mediator to the Father, as Our Lady is Mediatrix between Jesus and humanity.
It has very nice parts to that article, as a whole. Especially where it mentions that the Rosary consists of the Gospels! If you want defence, look to all the Popes, especially Popes such as Pope Paul VI and St. John Paul II, to validate your cause.
In terms of Scripture, in defending Our Lady, take your pick from these:
John 19: 25-29 - Mary as the Mother whom we are to devote ourselves to.
John 2: 1-12 - Mary as Mediatrix and Advocate at the Wedding Feast.
Luke’s Infancy accounts - Our Lady being the Mother of God.
…and also the accounts of ‘Jesus’ Mother and Brothers’ - Our Lady being the one who helps us back to her Son.
You also have the Catechism: