Note to Brad Pitt next time he tries a Canadian accent...

I am not so sure we disagree Jacinto, you may have misunderstood my argument. Maybe it was my Canadian accent, ha, ha. My point was that there was no way of knowing Bass' accent, but most older adults don't lose their accent, some do, some don't. Young people will lose their accent fairly quickly, often on purpose to sound like those around them. In 1850 there wasn't the ever-present media where we hear a country's standard English constantly in our ear. People would hear their family members mostly. People in one area would sound much more alike than today. Regional differences would be huge in a big country.
I would agree with one writer who said that the accent is not so important in a compelling film like this. We shouldn't complain about Pitt's accent since nobody knows what Bass sounded like. Freud said about small things creating the most estrangement..."narcissism of minor differences" ...like accents. People are different, which makes life more interesting, how boring if everyone looked and spoke the same. You say, "migrants demonstrate enormous variations in their accents".: My point exactly! Many keep their accents and never lose them. (We all have accents to someone else's ear.) I laugh when people claim they don't have an accent.
If everyone lost their accent easily, wouldn't migrants all sound "Aussie"? I was recently in Austria and said "G'Day" to everyone and no one said "G'Day" back, so much for stereotypes. (Sorry, love that Dumb and Dumber joke.) Did the brother and sister you mentioned arrive in Australia at different ages (likely young ages)? Bass arrived in the South in his middle to old age (for those days) over 42, a strong-willed, opinionated, stubborn man unlikely to change his mind or accent to fit in with those around him. He spoke out against slavery in an area where you could get shot for opinions like that. People put up with his argumentative nature because he was well-liked and a good-worker. This is the actual person Samuel Bass I am writing about.
Greg Norman at times sounds Aussie to a North American ear. You can hear it in the "r's" and he has been in the U.S. most of his life, not 3 years like Bass in the South. In 1850 most people sounded like their parents.
Yes, the accent is a trivial point and not a good reason to attack the film or Pitt. You can't please everyone. I was trying to point out the story of this actual, living, extraordinary person Samuel Bass who in his own small way did the right thing in a difficult time and place and changed history, an example for all of us, a hero. One person in their day can have a butterfly effect. Bass helped free Solomon who escaped slavery to write a book which helped the anti-slavery movement which was a part of the Civil War and the end to slavery in the U.S eventually.