Yes there is more than one theory of what the grail is/was. On one hand in the classical story is that it is the cup the Jesus used at the Last Supper.that one of the apostles took with him afterward and hid it.or gave to Joesph of Aramathia along with Jesus Shepard's staff.and they wound up in Ireland at Glastonbury Tor.
Another slant can be found in Maurice Cotterell's book Jesus, King Arthur, and the Journey of the Grail(The Secrets of the Sun Kings.)
Another is of course the story of King Arthur and the quest for the Holy Grail to reconnect the king to the land.
Yes Jesus' physical body was very human and it possible that while he was still alive he married Mary Magdalene . He was a Rabbi also and could have preformed the marriage ceremony him self.
The book your referring to is. the illustrated Bloodline of the Holy Grail(The Hidden Lineage of Jesus Reveled.)
The Question then becomes why should it change much of anything in the Bible.It won't ruin the real Christian Faith. Just the lies and added on rites and rituals that the Church took from other religions to try and get them to convert.
They would just have to go back and make it more along the lines of the Jewish Faith of which it is a Sept. not a whole new religion.
Jesus said before the accession that the Gentiles were to keep his story and words true and pure until the Jewish people truly believed that He was the True appointed Messiah.