Honestly, with Keldakar tethered to his home planet, I could see the GB set up in a way that has both an absolute leader, and also several lesser leaders. It's possible Mraize IS the leader of the Rosharan chapter, and he reports to their boss through his babsk (or, apparently, by interacting with his "avatar"). If i may don my tinfoil hat, I'd like to kick in the idea that Iyatil, whose identity seems rather strongly connected to southern Scadrial (even if she didn't grow up there) may have enough connection to Kelsier that he may be able to speak through her using some form of hemalurgy. This would explain the comment about the Avatar without invalidating him as a candidate.