I've brainstormed and come-up with a few checks which will ensure whether a character is a valid Rum-suspect or not. For some stuff where no evidence was given in Manga I've used Occam's Razor.
Let's call him X. Now from Kohji case we can deduce that Asaka wasn't Rum. If that was the case, Rum would have killed Amanda in a more convenient way, he won't make the murder scene a hotel. Also the dyimg message makes no sense if it's interpreted as Asaka=Rum. Now if really Asaka killed Kohji(not Rum) then Rum part wouldn't have come in the message.(anyway that's wrong as we know Rum killed Kohji.), even if Asaka and Rum were the same person how did Kohji know of Rum's Bo codename? Even let's assume that he heard it accidentally why would he waste his last breath on a vague code-name, when he has the more revealing name Asaka?(Using Occam's razor) Now here I present you a solution, Rum wasn't Asaka, but a person who had a working relationship with both Kohji and Amanda. Kohji knew of his name, Rum killed Kohji because I think Kohji saw him coming out of Amanda's room and as Kohji knew him(his social persona) he had to silence him. His latinaized/Romanized/Anglicized name contains "U MASCARA", and he holds a social position so high that he keeps such a secrecy in his BO dealings.(If people saw his face, then there wouldn't be 3 conflicting description of his persona.). As of now no suspicious character seems to fit these criteria which I've set.
But before applying this filter , initial Rum checks would be a)Prosthetic eye, b) Being accustomed with the said eye c) Suspicious behaviour.