I had answered this in another forum, but any comments are welcome.
The regrant of 176 did not extend the remainder to all descendants of the 1st Earl, but rather to the Duke's heirs of entail. According to Scots Peerage, Volume 7, the heirs of entail were those listed in the entail of lands from 1693, i.e. :
"The lands were entailed to a long series of heirs, which, as the later and more important entail of the dukedom was based upon it, may be given fully here. The lands were to pass to Lord William Douglas and the lawful heirs-male of his body by Jean Hay ; whom failing, to his heirs-male by any other marriage ; whom failing, to James, Earl of Drumlanrig, and the heirs-male of his body ; whom failing, to William, Duke of Queensberry ; whom failing, to Anna Douglas his daughter ; whom failing, to any other daughter of the Duke ; whom failing, to any person to be nominated by the Duke, and if there were no such nomination, then to the eldest daughter and heirfemale of Lord William Douglas by this or any other marriage, and the heirs-male of her body ; whom failing, to the eldest daughter of James, Earl of Drumlanrig ; whom failing, to the eldest son of Mary, Countess of Galloway, eldest sister of the Duke, or to her second and third or other sons ; whom failing, to the eldest son of Catherine Douglas, Lady Kelhead ; whom failing, to her second and remaining sons ; whom failing, to the eldest son of Henrietta Douglas, Lady Lag, third sister of the Duke ; whom failing, to her second and remaining sons ; whom failing, to the eldest son of Margaret Douglas, Lady Applegirth, fourth sister of the Duke ; whom failing, to her second and remaining sons; whom failing, to Sir James Douglas of Kelhead, and the heirs-male of his body; whom failing, to William Douglas of Dornock ; whom failing, to Duke William and his heirs-male whomsoever ; whom failing, to the Duke's heirs and assignees."