In one major way, Regina takes the road not traveled by Mrs. Alving. She doesn't bow to duty. "I really can't stop out here in the country and wear myself out nursing sick people" (3.142), she says matter-of-factly, once marriage to Oswald is no longer an option. Because of her working-class roots and Captain Alving's blood, she's not tied down by the strong puritanical feelings of guilt and obligation that kept Mrs. Alving suffering for so long. Her destiny may not be a noble one, but it's the one she chooses.
Pastor Manders needs Engstrand. Without a dusty, groveling carpenter to convey respect and confess his sins, how would the Pastor define his identity? The Pastor is gullible and Engstrand is savvy – but as we know from the pact they make at the end of the play, they are both equally dishonest.