He didn't die because of Kira, he died because he looked into a Shinigami's role. A better suggestion would be to think of who Behaku suspected since that's why he looked into them. We can then find the Shinigami.
Officer is the most common role. It is more likely for one to be an officer twice in a row than an officer then Kira/assistant.
If SK is not Kira, I don't see how that makes Yanks Kira.