Ugh. Paraphrasing we go.
Rowen and jail see each other, and Sayno confirms neither are evil. Neither are Arbiter, because otherwise Jarek’s ability would’ve failed. Except we know Jarek’s action succeeded because the Death arcana Jarek gave to carbonated was what killes Derps.
–except not really. Apparently the Investigative duo (Rowen and jail) are not confirmed because of Arbiter’s ability to fake visits as fake roles.
jail’s credibility as Investigative is solid because he claimed first, but what about Rowen’s? Rowen confirmed jail’s visit much much later because he “missed it”, and his claim only confirms jail’s existence. Compared to jail, Rowen is more likely to be Arbiter if there is an Arbiter among the two of them. (Realistically though, I don’t see HoB giving the Arbiter class to Kelsier. Like, it just doesn’t make sense.)