Darnell is not actually American, he was born in England but lived in America for a long while.
I'm not sure how Darnell would do in the US version BB, I think he cares too much about what people think of him and of those around him and in the US game you really need to let go of all emotions if you want to do well and not care who you stab in the back. I think Darnell would do fairly well as he has good perception on people but I don't think he'd be able to win the US show.
Not really a fan of the Aus version, haven't watched much, so not sure how Sara would do, I get the feeling it's pretty much like our series where flirty females aren't generally liked so Sara may have been evicted early in BBAUS.