The original source of the quote is this article:
The reference is to the location of the words in St. Gregory Palamas' writings in Patrologia Graeca. This is a widely available work which contains all his works. Why not look it up? I'll certainly do the same at our local Seminary library or maybe the Univ. of Penn. library if you really can show me these aren't his words.
While you are at it, St. Gregory is quoted again saying "Being mediatrix between God and mankind, she made God the Son of man, but she made every human creature a child of God" in this article:
Mary in the Mystery of the Church: The Orthodox Search for Unity by Prof. Vladimir Zelinsky
One of the great problems facing the Church is the growth of doctrine which was not part of the revealed faith of the Church.
It is indeed only speculation to delve too much into some of these issues, but the Orthodox church teaches that we pray to Christ, the Father, and the Holy Spirit, and we honor/venerate the Saints, with Mary being the most Holy and honored of the Saints.
In the long run, as an Orthodox priest told me recently, it is best to stay with Holy Scripture. The church canons and Holy Fathers are part of our Tradition in the church, but they are not laws. It is the spirit which is the most essential part of our theology, not the legalistic issues.
Yes, in *this* sense she was a mediator, because she brought Christ into the world. But that is different from saying that she mediates now between us and God.