Thanks, RJ. I'm looking at all angles. It's a tedious process. Angus could be related to Sir John, but I'm not sure how many generations back I will have to go to find the intersection of Macdonalds, and the records are already spotty on FamilySearch.
So far I have (all dates approximate from Ancestry.com matching results):
Donald Macdonald (1620-?)
Alexander Macdonald II of Balranald (1680-1760) = Catherine Maclean of Boreray (1710-1797)
Angus Macdonald (1749-1830) = ?
Captain Robert Macdonald (1779-1809) = Jean Gray (1790-1868)
Angus Macdonald (1808-1887)
… Angus 1808's mother Jean Gray married secondly John Smith (1786-1849)
their daughter (Angus' 1/2 brother):
Isabella Smith (1826-1905) = Joseph Ford (1832-1922)
Rowland Ford (1860-1932) [Note: his brother was Angus MacDonald Ford - a namesake clue, suggesting that the notable connection to Sir John A was through near kin] = Susan Maud Vosburgh (1863-1934)
Newton Ford (1890-1940) = Hannah Mitchell (1893-1980)
Gwyllyn "Glenn" Samuel Newton Ford (1916-2006) had several wives, notably Eleanor Powell (1912-1982), by whom he had a son who is still living, according to Wikipedia.
On the Sir John Alexander Macdonald line, from WikiTree, the furtherest documented ancestors start with:
John MacDonald (1736, Rogart, Sutherland, Scotland-1822) = Jean MacDonald (1757-?. Note that FamilySearch says that she, too, was a MacDonald)
Hugh MacDonald (1782, Pittentrall, Sutherland-1841) = Helen Shaw (1779, Highland-1862)
Sir John Alexander Macdonald (1815-1891) = Isabella Clark (1809-1857)
Hugh John Macdonald (1850-1929) = Jean Murray (1855-1881)
their daughter Isabella Mary (Macdonald) Gainsford had a son by an unnamed Gainsford (presumably George Kelvin Gainsford), but no descendants by the grandson Hugh Alexander George Macdonald Gainsford are listed by name (maybe living?)
Hugh John Macdonald (b. 1850) also married Gertrude "Gertie" Agnes VanKoughnet (d. 1940)
their son John Alexander "Jack" Macdonald has no descendants listed. as a source for Jack, "Children of the Prime Ministers of Canada" is listed, but apparently that was taken down from Wikipedia. The nearest category page doesn't list Macdonald survivors (Hon Sir Hugh John MacDonald, 1850-1929 is the only son who had surviving sons, if I am not mistaken).
In sum, it looks like the best chance of any sort of connection is among ancestors, vs. descendants of Sir John A. It's an interesting study. At some point, I may be writing to the obituary author. I did note that one of the FindAGrave borrowed biographies mentioned that family papers were used. . .