Could you help me to come up with a title for an essay that analyzes the story's plot structure and the main thesis of which is an answer to the question "How does Chopin use the climax of her story to make a point?"
I agree with copelmat that the final line of the story would make an excellent title. That final line is extremely ambiguous and perhaps even ironic, and so that title would give you the chance to take your paper in several possible directions. Indeed, that title would allow you to make the argument -- already made by Lawrence Berkove, among others -- that the story is not at all an endorsement of adultery but is in fact an ironic condemnation of the adulterous affair.
If you did want to argue that the story in some ways affirms the affair, here is another phrase from the text that might make a good main title (with an explanatory subtitle: "the very borderland of life's mystery." I could imagine a paper titled this way: "The Very Borderland of Life's Mystery: Sex and Awakening in Kate Chopin's 'The Storm.'" Even as I type that title, however, I find it a bit silly, because I personally think that Chopin mocks the affair as much as she admires it (if she admires it at all). The whole penultimate paragraph suggests that Chopin is writing in a very ironic mode:
As for Clarisse, she was charmed upon receiving her husband's letter. She and the babies were doing well. The society was agreeable; many of her old friends and acquaintances were at the bay. And the first free breath since her marriage seemed to restore the pleasant liberty of her maiden days. Devoted as she was to her husband, their intimate conjugal life was something which she was more than willing to forego for a while.
You are correct in suggesting that the plot structure of the story is essential. But the climax of the story is really only important as it allows for the denouement, the resolution of the conflict. Chopin summarizes this so very well in the story's very last line: "So the storm passed and every one was happy."
In my mind, that last line would be a great title for such an essay.
How about this one?
"Kate Chopin uses the torrid affair between Calixta and Alcee to illustrate her belief that passionate love is not synonymous with marriage."