First of all, we don't write essays for questioners, but we can lead you in directions that can help you write your own essay.
The question you ask: "What might have happened if the Whites had opened the door when the knocking occurred?" cannot be answered. The reason it cannot be answered is the point of the story.
The "Monkey's Paw" is about the open question of whether fate rules our lives. Neither you nor I nor the author nor anyone else knows the answer to that question. We all have our theories on the matter and no one knows for sure. No one. That's why the Whites do not open the door and why the reader is left with an image of an assumption without a resolution.
It is the sound of one hand clapping.
As to horror tales, in general, this story is of a special nature. The only really awful and horrific thing that happens in the story is that Howard gets caught, mangles and killed in the machinery where he works. The rest of the horror, if there is any, is in the reader's mind.