Lady Margaret's wound seems to move. When first hit, she presses her hand to her ribs, but when hiding it, she smooths her shawl carefully over her breast. Her breathing difficulties suggest the wound is indeed in the chest. Helen seems to be looking towards her bosom when she notices it, but there is no visible blood. The most plausible in-universe explanation is that the ball glanced off her stays and went up into the breast, but it's more likely poor continuity combined with Disney/Hays Code coyness over chest-wounds in women.
When Lady Margaret is wounded, there is a framed portrait of a man visible in the room behind her. From the style of his wig, it clearly dates from several decades after the setting of the story - probably 1780s.
Hamish Macpherson compares the Marquis of Montrose unfavourably with his ancestors, eliding his grandfather, James Graham, the first Marquis ("the great Montrose") with John Graham of Claverhouse, Viscount Dundee ("the bonnie Dundee") as if they were the same person. They were, in fact, only distantly related and overlapped in date only briefly: Montrose lived 1612-1650, Dundee 1648-89. It is unclear whether this is an error of the script or whether it is meant to indicate that Hamish's reminiscences are unreliable and overblown.