From your thesis, it is clear that you have a solid grasp on this fascinating story. I would, however, look at shortening your thesis slightly (specifically your description of your theme) to create a more clear singular point. Whenever a potential thesis seems too long, it is always worth seeing if you can transition some of what you've written in your thesis to either the introductory material leading up to your thesis or to your conclusion. Here are a couple of suggestions as you look at revising:

Firstly, since your writing prompt asks for identification of the theme and then for analysis of how the setup of the piece provides a vehicle for the theme, I would go in that order, while trying to look for a way to shorten the thematic part of your thesis statement. In the example below, I also included, instead of what you had previously as the general statement "the elements of writing," the specific qualities that you wanted to reflect on in the piece. Specificity is always a good thing in a thesis. Here is a slightly rewritten version of your original thesis:

In "Happy Endings," Atwood illustrates the theme that motives are infinitely more interesting than events through her development of plot, characters, and point of view.

Also, since this short story is such an excellent example of metafiction (meaning "fiction within fiction"), weaving that into your thesis would strengthen the uniqueness of your reflection. "Happy Endings" is a rarity in the short story world because of Atwood's choice to incorporate metafiction. If you wanted to go that route, your thesis might look something like this:

Throughout her short story, Atwood brings her reader to the realization that a list of plot points fails to have the depth that analysis of rationale does through her unique usage of metafiction.