I dunno, I have to agree with Roxor here. It's fairly obvious who did it, just listening to the conversations and studying the characters. That being said, no, there are no hard evidence, but that's actually a good thing, I think, because in quests of this type, I think that it's often far too easy to work your way to the hard evidence and then just make the accusation, presto, done. And that's extremely boring. I think that that quest is actually fairly well-written, but lacks an emphasis on clarifying that you don't have any hard evidence, but that hard evidence isn't necessary in this case, because we're operating on feudal assumptions in what amounts to a criminal underworld context, so what you say, your interpretation, is what matters, not whether you've got DNA evidence or not.
But I absolutely think that the correct answer is fairly evident, partly because the other two are "too obvious", and are very straight-forward in their dodgy dishonesty, and very honest with the fact that they're shitty people. Moon logic? Maybe. You certainly cannot reduce this to pure logic, but with some narrative/game familiarity and provided you can read characters and social contexts (also known as not being a raging autist), it's fairly evident who the killer is, once you've exhausted all options. It would've been a far more interesting plot-twist if one of the more obvious bad guys turned out
to actually be the bad guy. We've reached a level of post-post-irony in
the age of cultural hipsterism where it's practically impossible to expect anyone
but the not-villain to be villainous.
I would've had her run off if you actually leave the Dendra O'Hur to ask Fulsom if the alibi of the cult leader is actually legit (that he spent time with Fulsom), auto-pinning the blame on her, concluding the quest, but if you actually keep digging or when you catch up with her, reveal that it was one of the other two actual-assholes, possibly for completely unrelated reasons. Or something along those lines. I think that the quest as-is is too predictable and one-dimensional, especially as a quest connected to a would-be major arch/confrontation that comes later (which is tragically mishandled as well, but that's a separate issue).