I think that Stella is shown to be a very unique character in the film. I think that her defiance of Stanley at the end of the film is something that makes her character so unique. After a prolonged period of seeking to bridge differences and acquiesce to Stanley's ways, the end of the film shows her taking refuge with Eunice and leaving her husband. I think that this is significant because to shows how Stella has come to realize Stanley's malevolence and cruelty. Her actions show how an individual must take a stand against that which is wrong no matter how uncomfortable it may be. Stanley's cries of "Stella" can be seen as a desperate attempt to hold onto that which is gone or could even be seen as a final understanding of his own savage nature. This is not as evident in the play, and in the film, the character brings more of a structure or ethical order to the New Orleans setting. In this, Stella's character is a reminder to all that individuals do have power and autonomy and can change their reality if they embrace the moral awakening within them when they understand the need for make right that which is wrong.
I completely disagree with you, Stella will go back to Stanley and the audience knows this at the end of the movie. Recall the scene after the Poker Game, Stella and Blanche escape top Eunice's, and almost imemdiately Stella returns to Stanley when he calls out for her. It is all a part of their sex life. Stella will never leave Stanley and the film closes on a bleak note as the audience knows she will be straight back down to him again.