Re: John Grant of Lachine being a nephew of Captain Alexander Grant b 1734 son of Patrick Grant and Isobel Grant. (1) Patrick Grant is 7th of Glenmoriston, Isobel Grant dau of John Grant of Craskie.
(2) being a nephew of Alexander Grant would tell us a brother of Alexander had a son John.
From my listing I am having some difficulties seeing a nephew John born within the timeframe we need. Alexander's brothers are (1) John Roy killed at Flanders, (2) Patrick 9th Glenmoriston had a son John 10th Glenmoriston 1758-1801 remained in Scotland, (3) Captain Allan 1730-1808 do not know if he had a son John but did have a dau Barbara, (4) Alpin known as Major Alpin no son John listed in family, and (5) Lewis died in Jamaica.