Regarding the problem that some people have with Angelos being Chapter Master but not Chief Librarian, I felt that that was cleared up by his speech in the end about the new Blood Ravens being a chapter that honors the Emperor instead of being wrapped up in enigma. It seems obvious to me that they've changed their ways at least to some extent, although they obviously will still have tendencies dictated by their gene-stock.
Tarkus's speech when he unmasks actually makes me think that the traitor that he was referring to was the force commander from the previous games. That's just my impression though.
