A few more children showing potentially for Alexander (and Margaret Duff? but name not given):
From the list you showed Diddy:
22/12/1805 Mathew father: Alexander Abernathy
11/2/1810 Janet " "
8/5/1814 Robert " "
14/3/1824 Jean " "
18/6/1826 Andrew " "
8/11/1828 James
There is also showing in Abernathy OPRs:
BARBRA SPEEDIE Christening: 30 AUG 1807
MARGARET SPEEDIE Christening: 23 FEB 1812
ALEXANDER SPEEDIE Christening: 13 JUL 1819
Missing the entry for Marion born c. 1816.

Guessing game now with Scottish naming pattern and order:
Alexander - parents Matthew and Janet...maybe!
Margaret - Robert and Barbara...which fits with what we are seeing elsewhere
Monica