I believe that she did know. But kept these things in her heart.
Well, the song doesn’t make a statement from Mary; it is more a questioning from us. Also, I can’t imagine Mary or anyone who was dwelling on earth can understand and see God in His fullness, as the song describes Him - the Great I AM, the Lord of all creation, walking in Eternity with the Angels, Ruling the Nations.
So doctrinally, I think the song is pretty sound. It doesn’t give me any real problems.