Chekhov is considered a naturalist in that he tells of life as it is in a universe that is indifferent to individual human fates. He is also noted for the way in which his texts points to events "beyond" the narrative, leaving the reader with the feeling that a story will continue past its ending on the page—a lack of closure that imitates how life often is. Naturalism also depicts nature in an unidealized form.

The Cherry Orchard is naturalistic in the detail in which it shows nature, as well as in the unidealized way it depicts the natural environment. For example, although the characters in the play idealize and magnify the importance of the cherry orchard, the play itself shows telegraph lines cutting indifferently through the landscape: Chekhov does not present an idyllic or romanticized backdrop for this orchard—even if the family idealizes it.

Thematically, too, the play is naturalistic in that the forces of society doom the Ranevskaya family to losing their estate and their cherry orchard. The larger forces of the world are indifferent to any nostalgia or sensitive emotional attachment the family members have to the land. As the play points out, just as the family once indifferently let the serfs plant the vast orchard and didn't mind owning human beings, now a new group of people on top are equally indifferent to the fate of the Ranevskayas.

Since we are only allowed to answer one question at a time, I edited your multiple questions and will answer the main one: Is Cherry Orchard considered a naturalist piece of literature?

The answer is yes. The Cherry Orchard represents naturalism in that it presents the reality and inevitability of what life can bring onto someone who has not made right choices, and will present to you with no aesthetic or stylistic devices the cause and effects of things. Naturalism also indicates the presentation of characters who suffer from personal deficiencies and problems that lead them to their fates. You see that even Firs, the poor manservant,  was so behind his times that he was literally left behind when the family moved.

However, it is in the sadness that comes with the cutting down of the cherry orchard that the naturalist aspect of the story comes. All is measured, decided, planned-ahead, and will be carried on. No matter how much love or decadence the orchard represents, its symbolism and beauty will not dictate its salvation like it would in a neoclassical or pre-raphaelite work. If it has to go down, it will go down and no matter who cries, despairs, or suffers as a result, life is life, and all is fair.