I would venture to say that part of your challenge is that it is difficult to link Williams' work to naturalist ideas. Take this fundamental description of Naturalism: "Naturalist writers believe that truth is found in nature, and because nature operates within consistent principles, patterns, and rules, truth is consistent." If we apply this to Williams' depiction in Streetcar, we can see that not much of this is present. There is little structure to the world that embraces Stanley, Blanche, or Stella. If there were objective rules and structures, it would be easier for these characters to function. The challenge that Williams brings out is the idea that the modern setting is one where freedom is the only absolute. Yet, with individual conceptions of freedom comes ambiguity, uncertainty, and doubt. These ideas are the diametric opposition to scientific patterns and rules. We could use the ideas you pose, though, to help explain some of the characters' motivations. For example, Blanche is trapped by her own genetic background in being a belle from the "old South." This crushes her because she is incapable of being able to adapt to how the new South functions. Stella's environment has had profound effects on her character in that she has become more pragmatic and practical concerning life and how it is led. Finally, I would say that Stanley is a product of his circumstance, a setting where white men find themselves having considerable more power than others around them. It is this power that enables him to rape Blanche and not endure social repudiation for his actions. In this light, we can apply principles found in Naturalism to Williams' work. Yet, I think that there might be some challenges in trying to assert that Streetcar is a naturalist work.