Thank you, Gadget -- my, that was quick! I know of no other children from James Johnstone and Mary Imbrie or Jonice or Jarvie, and because I've never been able to work out what her name was, I haven't been able to find out more about her.
James Johnstone of Selcoth and his wife Anne Dalrymple (who were married 3 Dec 1765 in Edinburgh) had the following children:
Prinence, b 4 Sept 1766
Adam, b 17 Nov 1767
Robert, b 22 Feb 1769 (but is he "my" Robert?)
John, b 5 Feb 1771
Adam, b 17 May 1773
Presumably the first Adam died (since another son is named Adam), so Robert might have inherited the large farm of Selcoth (nr Moffat). In that case, why was he a Merchant in Moffat? That's one thing that makes me suspect we're dealing with two Robert Johnstones. On the other hand, Selcoth was sold out of the Johnstone family in 1794, so that might explain a change of career or decline in family fortunes.