About the accents.. I was thinking while watching the film, how strange it was to have the two leads in a film about the American War of Independence both played by two Aussies. Not being a native English speaker myself I was utterly unable to judge how well they spoke the proper accents for the roles, but knowing Heath I'm sure he did so to perfection!
Actually, Mel Gibson is American. He was born in Peekskill, NY. His mother was Australian and they moved to New South Wales when he was around 11 or 12 (I think. I can't find an accurate source right this second). His acting career began in Australia and he picked up an Australian accent for the Mad Max movies and Tim, but seemed to lose it pretty quickly when he re-settled in the US and moved his career to Hollywood.
I suspect that at the the time of the American Revolution, all the colonists/revolutionaries/patriots (whatever you want to call them) all spoke English with a decidedly British accent. I doubt that southerners sounded southern and "y'all" probably hadn't been invented.
L