Bruce and Edward would have spoken a dialect of French now known as Anglo-Norman French, which was, to a large extent, the spoken language of the higher social strata in medieval times in Britain. As such Anglo-Norman French was commonly used for literary and eventually administrative purposes from the 12th until the 15th century. Therefore it really does not matter what accents are used to portray these characters in films, and even more idiotic for anybody to criticise foreign actors attempting Scottish, or whatever accent (Thinking here of Mel Gibson in Braveheart). Irrelevant what the Scots of the time spoke as Bruce wasn't a Scot.

As you yourself point out Mary Queen of Scots was of the 16th Century, therefore from a period where "English" was the primary language of mainland Britain. She left Scotland when she was five years old and lived in the French Royal Court where she most definitely would have spoken French and while in France she was taught Italian, Latin and Greek. She had four Scots companions to whom she would have spoken Scots. Her son James, who became James VI, never left the British Isles and as far as I am aware neither did Mary's Father James V.

Rather liked this though Alan, it did make me chuckle:

.... it seems sensible that they'd be given modern Scottish and English accents to differentiate them in the viewers eyes!

I'd have thought their names alone would have been good enough for their ears as a means of differentiation.