To prevent the magus from dying you have to make sure to move the prisoner to the marabad prison and NOT keep them in the palace dungeon. To prioritize the public do things like opening the coronation up to the public, pulling the curtains back on the way to the coronation, stuff like that. Also I don't remember completely but if Enzo was the one who gets injured when the assassin escapes then you have to say yes to adding extra security during the coronation. When I did that Enzo only had a broken arm, there might be a route where he isn't injured at all but I don't know it :/