How factual is the story behind the holy grail as depicted in the da vinci code? Was Mary Magdalene Jesus Christ's wife? Is the Holy grail really considered to be a female? Did constantine really merge pagan and christianity to form modern christianity?

First off, the legend of the Holy Grail was invented in c. 1170-1190 by the French poet, Chrétien de Troyes. It was not heard of before then. So any attempt to backdate it more than a thousand years and change what the words mean is ... stretching too far. Oh, no doubt the cup of the Last Supper would be a great relic that everyone would have wanted to find like the holy lance used to kill Christ. But we don't have it put in mystic form until Troyes and the five authors who wrote different endings to his unfinished story.

None of this Sang Real/San Grail theory shows up until after WW2. I know, we only got smart enough to figure it out then! There have been several "Da Vinci busting" shows that deconstruct the whole theory, in one aspect or taking on all the threads of supposed evidence. I particularly liked The Real Da Vinci Code with Tony Robinson

Basically, if you draw out the various elements behind the theory you find that actual historical or traditional information has been twisted or spun to create a structure that never really existed. The guys behind the whole non-fictional Holy Blood, Holy Grail are shown to be dishonest, the tradition of the Marys bringing Sarah ashore in France are shown to be about different Marys coming ashore with their servant, Sarah the Egyptian. We won't even get into the blood pressure changes the whole Templar line causes in Templar scholars (a friend of mine has a PhD in Templar studies, and don't even say the name da Vinci around her any more).

So I think we can say, no, legitimate historians do not consider this whole San Grail/Sang Real business good history. It's fringe stuff, like ancient alien astronauts.

EDIT: removed unnecessary remark