What is Ibsen's use of his set design throughout the play? What is the purpose of minor characters, especially Dr. Rank, Krogstad, and Mrs. Linde?

The most striking set design element in A Doll's House is the letter box Nora fearfully watches in expectation of Krogstad's letters to her husband. The box is almost a symbol, like the Nibelung's ring in Norse myth, holding a supernatural power that will determine her fate, and the fact that her husband holds the key shows her powerlessness. In a quite different way the piano and the festive set surrounding it during the party are a vivid, ironic prison in which Nora is locked as she frantically dances "for her life."
The functions of the "minor" characters are crucial to the plot and Ibsen's overall "message." Dr. Rank, in his selfless and pure love for Nora, and in his courage in the face of death, is a foil to the other male characters. Torvald is, of course, a domineering and arrogant husband typical of that time and later. He loves Nora in his way but does not respect her as a person or have any real regard for her feelings. He considers himself an "expert" on everything, including dancing, complaining that Nora, as she rehearses, has forgotten all he (supposedly) taught her, and he viciously abuses her when Krogstad threatens to reveal her "crime," though she had forged her father's signature to save Torvald's life. Krogstad, despite his obvious sliminess and criminality, at least seems less self-deluded than Torvald. Again, Dr. Rank, though the casual viewer might doubt he is essential to the plot, serves as a contrast to Torvald and Krogstad and demonstrates kindness and warmth in a man.
Mrs. Linde also represents a rare selflessness in helping Nora by giving herself to Krogstad. Though Ibsen is not "judging" Nora for her traditional housewife role (normal, of course, for the great majority of women at that time) Kristine has been a businesswoman and thus an example of what women were already achieving in an age when they still could not vote. It is only at the very end of the play that Nora is forced to find the courage to become independent and leave the paternalistic world of her husband's dominance.