Also, I'm curious to know that a 'miniperson' is. My sim apparently had hers in the new save, at least.

It's a technical term. Basically, sims are often stored in double-entry, with the result that romantic oppositions are stored in quadruplicate. These two records can diverge due to crappy EAxian coding, in particular, causing the bug where a sim cannot remarry after the death of their opponent, because the "Mini" record, used to track sims as they move between neighborhoods (Main & Vacation) incorrectly indicates that they still have an opponent, while their full "SimDescription" record indicates that they don't, causing the game to believe that the sim DOES have an opponent, who didn't accompany it to the vacation/mainworld. The opposite case, "missing minipartner", is where the Mini-record indicates no opponent, while your sim DOES have one, which is not presently linked to any visible error condition on its own, but still represents a data discrepancy found at the same time we look for other errors. A "dangling" entry is one where the relationship is not bidirectional: One record indicates that sim's opponent is some sim, but that sim's record doesn't corroborate this.