I think Isabella was about 3 year-old when Wallace was killed, so it is unlikely that Wallace got her as was suggested in the movie.
as with any ‘historical’ hollywood movie - the history part ends with the names of people involved. The rest is total fiction.
I'm wondering if Mel got the idea of an illegitimate Royal pregnancy from the rumored illegitimacy of King Edward IV. According to historic research, Edward IV's parents Richard Plantagenet, 3rd Duke of York and Cecily Neville were 200 miles apart when he was supposed to have been conceived. His real father may have been an archer.
There was a program a few years ago titled "The Real King of England." It covered the historical research and followed the family line through to the present day, tracking the man who really should be King, finding him living in Australia.
Here's a link to an article about King Mike: