I would be interested if you can find something new on the Killen-Macaulay connection.
In the meantime, here are extracts from the Ulster Directory of Doctors on Lurgan Douglases.
DOUGLAS, WILLIAM S. (d 1842), Naval Medical Service and Lurgan, county Armagh;
LAH DUB 1809; joined the Naval Medical Service as assistant surgeon; apothecary and surgeon of Lurgan; member of the Belfast Medical Society from 1825; died 8 May 1842 at his home in Lurgan. [Apothecaries (1829); BNL 13 May 1842 (d); Bradshaw (1819); Malcolm (1851); Pigot (1824)]
DOWGLASS, WILLIAM (1742/3-1812), Lurgan, county Armagh;
Born 1742/3; surgeon and apothecary, of Lurgan; died 1 January 1812; “in the arduous duties of a professional life was honoured with the esteem and confidence of a generous public.’ [BNL 7 January 1812 (d)]
DOUGLAS(S), JOHN CUPPAGE (1778-1850), Dublin;
Born 1778 in Lurgan, county Armagh, son of a general practitioner of Lurgan; served 5 years’ apprenticeship to his father; studied medicine at Trinity College, Dublin, and Edinburgh; LRCSI 1800; LAH Dub 1800; MD (St. Andrews) 1803 (on testimonials); LKQCPI 1810; FKQCPI (hon) 1832; surgeon to the Militia Regiment of Foot in county Tipperary; obstetrician in Dublin; assistant master of the Rotunda Hospital 1808-12; with Joseph Clarke he ‘laid the foundations of the high repute of Dublin as a school of midwifery’ (Wilde); of 16 Rutland Square East; author of various medical papers; married 23 January 1817 in Lisburn, Elizabeth Fulton (who was born 1783), second daughter of Joseph Fulton, solicitor and merchant, of Lisburn, and Ann Graham of Lisburn; died 20 November 1850 at 16 Rutland Square East; probate Prerogative Court 1850. [Apothecaries (1829), BNL 28 January 1817 (m); Croly (1843-6); Crone (1928); Dub Nat Arch, Prerog Wills index; Hope (1903); Kirkpatrick Archive; Kirkpatrick and Jellet (1913); Newmann (1993); O’Doherty (2002); Smart (2004); Widdes (1963)]
Is John a son of your Dr William Douglas and Margaret Cuppage, if he’s about the right age?