The excerpt that tells the White family's uncertainty on the Major's story would be when Mr. White said:
"If the tale about the monkey's paw is not more truthful than those he has been telling us," said Herbert, as the door closed behind their guest, just in time to catch the last train, "we shan't make much out of it." This is because they are not sure what is true about the information that a stranger told them in the train.
Its a prepositional phrase because it doesn't have a verb. If you have any more question feel free to ask me.
It could be a metaphor since you are comparing them to reptiles without using like or as.
