"Correct me if I'm wrong, but I believe most of what we know of William Wallace comes from a ballad by a minstrel called Blind Harry that was written after the fact."

Yes Blind Harry's epic poem about Wallace was written in the 15thC and was written in Scots. Hamilton of Gilbertsfield did an English translation in the 18thC and this was the work Randall Wallace based much of his novel on though he also supposedly interwove some of the emements of story of Christ into it. Right from Harry's version it was a swashbuckling adventure full of imagination, exaggeration and even distortion.

Some of the things that get most criticism for being 'made up' by the film makers were actually taken directly from the poem or adapted from the poem. For instance the blue face of Wallace! But in the poem Wallace is visted in a dream by a fairy (in the later version the Virgin Mary) who crosses his face with a Saltire. The liason with the Princess in the film is much criticised but again this is simply adapted from the poem except in the poem it is the English Queen herself who has the liason with Wallace.

One of the big differences between film/book and poem is how the Irish soldiers are portrayed which I suppose is a sop to Irish Americans. In the film when the Irish serving under Edward of England actually come face to face with the Scots they down weapons and both sides start hugging each other in some kind of Celtic solidarity whilst the English king simply shrugs his soldiers and whispers something like "bloody Irish". However in the poem when Wallace's army catches up with the Irish and Scottish Highlanders serving under Edward he spares all the Highlanders who swear allegiance to him (because they are fellow Scots) but all of the Irish are massacred on the spot for being foreign invaders.