Good question! I think you're very right in stating that this is a deliberate choice on Chopin's part. The focus of this story is upon an hour which completely changes—and ultimately ends—the life of Mrs. Louise Mallard.

As a wife, Louise is defined by her husband. She is trapped in her role as Mrs. Mallard to the extent that she feels her own identity has been lost. The choice of name is perhaps deliberate too. A mallard is a type of duck; Louise has been made something of a lame duck through her marriage. She has forgotten that she has the capacity to fly in and of herself.

The motif of the window is repeated throughout the story as Louise looks outside at the prospect of freedom. She is at first terrified by the idea of what will happen to her now that her husband is supposedly dead; but as time goes on, she comes to realize that what she is feeling is not fear after all but anticipation of a new life in which she will no longer be defined by her husband and can actually be free. As a widow, she would still be able to use the benefits of being Mrs. Mallard socially; she would be much freer than she had ever been as a single woman.

At the end of the story, Louise is so shocked to find that her husband is not dead that her heart stops. She dies as Mrs. Mallard after all.

Why does Chopin call her protagonist as "Mrs. Mallard" rather than by her first name?

In the first sentence of the story, Louise is referred to as Mrs. Mallard:

Knowing that Mrs. Mallard was afflicted with a heart trouble, great care was taken to break to her as gently as possible the news of her husband's death.

Reading the sentence, we can see this opening line reflects the point of view of the women with Louise. They are the people thinking about the need to break the news gently to her.

To the other women, Louise is primarily defined by her marital status. She is Mrs. Mallard to them because it was customary in those times to think of a woman as an appendage of her spouse rather than as an individual in her own right.

It is only after Louise is alone by herself in a room that the point of view shifts to her own thoughts and feelings. Outside, in the space where her friends and neighbors are gathered, she is defined by her role as wife. However, when she is by herself she is gradually able to shed this role and begin to define herself as individual entity in her own right. As she realizes that she is free to be who she wants to be and no longer has to defer to her husband's wishes and needs, she begins to feel free and joyous. She is liberated—or so she thinks—to be Louise rather than Mrs. Mallard.