No shortage of Thieves in Riften... Although there was no Thief corpse, and we were relatively far from the canol, the Guards certainly don't have a 100% kill rate. I have to infer, since I found the Report note in Bolli and Nivenor's house, that the trigger event was from them. Dunno, though, Ingun was sketchy. Maybe she was trying out some nasty potion on Edda? As a cranky short-timer guy used to say over in Asia, "Yah don't know, do yah? Makes yah wonder, don't it?"
I had done the little quests for most of the dead folks. Ingun's ghost is still brewing me some potions. Next time, I'll back up the tape (this stuff happened when I had just gotten done with either Kill Mercer or Return Skeleton Key, so a do-over didn't occur to me).
Aaaand, now that I have the Nightingale Subterfuge spell, I could start some stuff--requires an enemy, I think, but maybe not. I won't, though. As noted up above, the NPC population is sparse enough already.