In short: I have unintentionally ensured that a honey-sipping-turned-masterful jewel thief will never trouble me again; the help page linked in-game claims that "there are ways to get back into almost every story;" I can find no information on the wiki as to how to do so in this case.
Is there any means to reset the jewel thief’s storyline, beyond deleting and restarting an account dating to 1893?
Did you divorce him or betray him?
I believe he is talking about the opportunity card where if you ignore him he slips a diamond into your pocket but you can also “get rid of him” by tying his shoelaces together.
The quality is called “Bitter parting”, and no, you cannot reverse this choice.