I would say that the family came out sometime after 1786 as that is when Marjory's nephew, John Grant Duldreggan, + children arrived in Montreal. Alexander died in 1791 as noted on the tombstone. Look for ships between 1786 and 1791.
There was a ship that came out in 1786 with many Mcdonells on it. It arrived about the same time as the ones whom left in 1785 and came into Canada via New York city and overland to Canada.
One of my 3rd Gr Grandfather Alexander McMillan came out on the 1786 ship, another, John Grant Duldreggan, came out on the 1785 ship which went in at New York City.