... to consider more generally how Austen handled the romantic attraction issue. The recent discussions of Dr. Gregory's ideas provoked in me the following considerations.

NA:
Henry responded in gratitude to Catherine's apparent affection.
Isabella responded in gratitude to Frederick's apparent affection.
James responded in gratitude to Isabella's apparent affection. (?)

P&P:
Jane responded in gratitude to Bingley's apparent affection.
Lizzy responded in gratitude to Wickham's apparent affection.
Lizzy responded in gratitude to Darcy's apparent affection.
Collins responded in gratitude to Charlotte's apparent affection.
Lydia responded in gratitude to Wickham's apparent affection.
Georgiana responded in gratitude to Wickham's apparent affection.

S&S:
Marianne responded in gratitude to Willoughby's apparent affection.
Marianne responded in gratitude to Col. Brandon's apparent affection.
Robert Ferrars responded in gratitude to Lucy Steele's apparent affection.

Emma:
Harriet responded in gratitude to Mr. Elton's apparent affection.
Harriet responded in gratitude to Mr. Knightley's apparent affection.
Harriet responded in gratitude to Robert Martin's apparent affection.

Persuasion:
Louisa responded in gratitude to Capt. Benwick's apparent affection.
Louisa responded in gratitude to Capt. Wentworth's apparent affection.

I've not gotten to Lady Susan or Mansfield Park yet (it's less than a year that I've been hooked on Austen).

There are others for whom I can recall no guidance in figuring out who responded to whom, and still other couples I'm sure I've forgotten; but it does seem that Austen at least usually followed the pattern proffered by Dr. Gregory.