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.
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.
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.