Covering some of the other points you have asked about, let's look at the climax. In "Lamb to the Slaughter," the climax occurs when Mary murders her husband, Patrick, with a leg of lamb. This occurs just moments after Patrick's shocking divorce announcement, which leaves Mary reeling and builds the tension. As for the resolution, it could be argued that there isn't one because Mary feeds the murder weapon to the police detectives, enabling her to get away with the murder.
Finally, the mood of "Lamb to the Slaughter" is subject to change. At the beginning of the story, for instance, the mood is calm and almost dream-like. Mary is waiting patiently for Patrick to return and has made sure that the house is in order to receive him. But the mood changes and becomes violent and hostile when Mary murders Patrick. Dahl descriptively illustrates this change:
"The violence of the crash, the noise, the small table overturning, helped to bring her out of the shock."
By the end of the story, however, the peaceful mood is restored as the detectives sit in the kitchen, pondering (rather ironically) what has become of the murder weapon.
We don't have room here for me to cover them all, but I will get you started.
Internal conflict is best shown in Mary. She is at war with herself when she finds out her happy home is not happy and about to disappear all together. She solves this conflict by killing her husband. Then she has to figure out how to hide the evidence, also an internal conflict.
The external conflicts are Mary vs. her husband and Mary vs. the police. In both situations, Mary must defend herself against what she views as an attack. When her husband says he is leaving, she fights back by killing him. When the police arrive to investigate, she fights back by deceiving them.
A metaphor in this story is in the title itself. Mary is a lamb due for slaughter - her happiness is slaughtered by her husband, even though she is innocent. The lamb that she uses to kill her husband is a metaphor for herself.
The tension in this story comes from dramatic irony - we as readers know that Mary is guilty, and are concerned that the police will find out. We also know that the police are eating the murder weapon, even as they discuss what the murder weapon is - more tension and more irony.