Unfortunately you have asked too many questions here - remember, enotes specifies that you are only allowed to ask one question, so I have edited this question down for you to focus on the ending of the story and the resolution of this excellent and chilling ghost tale. Clearly I am going to advise you to read it as merely reading my answer to this question isn't going to be good enough for you to get the grades you want, but here goes my answer anyway.
The ending and resolution of the story focuses on the second wish that the couple make to have their son alive again. When they hear a knocking on the door, they realise that their wish has been fulfilled, however, interestingly the reactions of the parents are very different. The mother is desperate to let her son in, but the father doesn't want her to do so. When the mother needs help to open the door, he is involved in doing something completely different:
But her husband was on his hands and knees groping wildly on the floor in search of the paw. If he could only find it before the thing outside got in. A perfect fusillade of knocks reverberated through the house, and he heard the scraping of a chair as his wife put it down in the passage against the door. he heard the creaking of the bolt as it came slowly back, and at the same moment he found the monkey's paw, and frantically breathed his third and last wish.
One of the many marks of excellence of this story is that we are never told what this wish is, but the empty street outside the door makes it clear that the father has wished for his son to return to his grave. Note how in the quote above we see his thoughts and terror about what they might find if they open the door. He refers to his son as the "thing outside", showing that he recognises that a son resurrected from the grave would be no son at all. The lesson of the story seems to be that getting what you wish for is very dangerous indeed.
It sounds like you haven't read the story. It's a shame, because "The Monkey's Paw" is an excellent short story.
SETTING. Laburnam Villa, an out-of-the-way home in a small English village.
CONFLICT. Sergeant-Major Morris has told a story about his mysterious monkey's paw, which supposedly grants each new owner three wishes. He tosses the paw in the fire to destroy it, but Herbert, the Whites' son, saves it. Morris urges them to rid themselves of the paw.
EVENTS. After Herbert saves the paw from the fire, the Whites keep it and begin their sting of wishes. Their wishes do not turn out exactly as planned.
CLIMAX. After receiving terrible news concerning their son's accident at work, the Whites are further horrified to find that their first wish has actually come true.
ENDING. After making a second wish that apparently does come true, a knock is heard on the door. Realizing that their second wish may have come true in the most horrible manner possible, Mr. White quickly uses his third wish. The door is opened--but no one is there.