Compare and contrast the imagery used in each scene when the sisters learn of their husbands’ survival. How does this imagery relate to the structure of the story?

The imagery is actually quite similar between the experiences of the two sisters-in-law. In both cases there is a knock at the door and a man appears to announce the news that their respective husbands, contrary to the previous reports, are alive. The first sister-in-law, Margaret, however, receives the word from Goodman Parker, a well-dressed man in a "broad-brimmed hat and a blanket coat," while the other messenger, telling the news about Mary's husband, is "a young man in sailor's dress, wet as if he had come out of the depths of the sea." We are also told the sailor is a former suitor of Mary. In addition, the night is totally dark when the first man appears, but during the second visit we're told the moon is up and a storm has ended.
In my view the similarities between these visits outweigh the differences. It's as if Hawthorne deliberately sets up an almost exact parallel to emphasize the sameness of the situation of the two, and then to observe each one's sympathetic view of the other she does not know has also received (or will receive) the reversal of fortune (in a positive way).
One interpretation of the story I have read is that one of the women has merely dreamed the visit. Or, one might perhaps ask if both visits have been mere dreams or hallucinations. I don't see any solid evidence for this view. Despite the Gothic and occasional supernatural elements of Hawthorne's fiction—and the latter are usually presented ambiguously anyway—his narratives are generally straightforward. The focus here is on the irony of each woman receiving essentially the same good news, but believing the other is still the unfortunate one.