A very interesting thread and by no means one with an easy solution.
Shakespeare's history plays already mentioned, mostly myth and/or made up by the writer to please his patrons, but how do the majority nowadays perceive the likes of Macbeth and Richard III?

My own gut feeling is that a film (unless it's a documentary) is fiction or faction, and to a degree the writer has almost carte blanche. If it's loosely based on history there has to be some semblance of fact usually to add to the realism. Do songwriters have any more duty to tell the truth as film writers? Or for that matter other genres? In an historical novel we would expect there to be some facts, but at the end of the day the writer has to fill out the story and sell books. IMHO many songs are written with a commercial purpose in mind (Sorry Jim!) and if you are going to sell your song you've got to make it interesting in some way. One way might be distorting the facts.

As I said, there is no easy answer.