I'm sure I read that when she audition the role was English and she read the lines with both English then Scottish accents, at which point they changed the charater to a Scottish one. It does suggest her having a Scottish accent isn't overly plot relevant, unless they rewrote the story line to make it so.

Also, in terms of Scots hanging onto their accents, one never need look further than Sean Connery... who has played people from all over the world and always with a Scots accent