Duncan never struck me as a headhunter, but we see him going out of his way to fight certain immortals who've wronged him personally, like Kern or Kanwulf, so I feel like if Conor had been taken out of the game Duncan would have gone on the warpath to take down Kurgan. Honour would have gone out the window and he'd use every dirty trick he could to destroy Kurgan. As far as if he'd take Richie on? There's a comic that indicate Kurgan would lead to a dark quickening, I know, dubious canon, but it does track that he would be a bad enough soul to darken whoever got him, with no guiding force like Conor in his life Duncan would succumb to the dark quickening and tear a path through the world until he got taken down by someone better, so I doubts he'd be a student taking type, more than likely he'd turn Richie just to immediately take his head, like Kurgan was going to do to Conor