The Monsignor was a fool for not believing Sister Jude's suspicions about Dr. Arden, but eventually he did finally see through Arden's sick and twisted persona when he saw Shelley in the hospital. Instead of doing the right thing, he kicked Sister Jude out of the asylum.
As for Sister Eunice, I realize she's possessed by the devil, but what the
is she trying to accomplish aside from killing people? I know she hates Sister Jude, so she threw a wrench in her plans by killing the Nazi hunter and trying to prevent her from exposing Arden's real identity. But it's not that she likes Arden, either. So what are her motives?