There's so much to ponder in Ch. 31! On this read, I was thinking how much Willoughby has in common with what we know about Colonel Brandon's older brother.
**Willoughby inherited his estate and apparently lives beyond his means. Colonel Brandon's brother did the same. He was in debt when they needed him to marry Eliza for her fortune and he had run the place back into debt again by the time Colonel Brandon inherited it five ago.
**Willoughby 'stole' from Colonel Brandon his chance at being with a woman he loved. So did Brandon's brother. The difference, of course, is that Marianne was a willing party to it, but Eliza wasn't.
** Both the elder Brandon and Willoughby seemed to have no idea of making their way in the world beyond whatever they inherited or took from a wealthy wife. With no profession and nothing to occupy their time, they find less than admirable ways to fill their time: Willoughby has expensive habits, and his pursuits such as hunting, horse breeding, living the high life in London, having a new curricle, etc. that are probably beyond his £600 a year income. Brandon's brother evidently had interests that were more sinister or unsavoury in nature, since he can only describe them to Elinor as "not what they ought to have been".
**Colonel Brandon is finding himself picking up the pieces after both of these men have used and discarded young women.
How this must hurt the colonel to be reliving this all again!!