What is the theme of The Bus by Shirley Jackson?

The Bus by Shirley Jackson is a dark tale with an ambiguous ending. The reader's interpretation of exactly what happens at the end is crucial in determining the story's theme. Is Miss Harper suffering from mind-altering effects of her sleeping pill? Did any of the subsequent events really happen, from the truck ride to the closet from her childhood? Or has Miss Harper become stuck in an endless circle of her own psychological breakdown?
It could be argued that Miss Harper has suffered from some type of break with reality from the beginning of the story. Using that as a basis for thematic development, one possible theme is that even when all sense of reality is lost, the idea of home brings comfort.
After all, Miss Harper longs from the beginning to go home. She is desperate (and angry) to get there, spilling her frustration out onto all who cross her path. She meets various characters along the way, and she treats them all pretty badly, even writing letters of complaint in her head. And she dreams of home. A warm bath. A cup of tea. Her own bed.
Yet this home isn't where she ends up. Instead, she is taken to what evolves in her mind as the home of her childhood—the place where everyone finds that fundamental, early sense of security. Her childhood doll comes to life and chats with her. (Unfortunately, the doll calls her an old lady, which horrifies her.) She can hear her father on the piano and her mother singing.
And suddenly, she's back on the bus, again being dropped off at Ricket's Landing to start the entire cycle over again.
Maybe Miss Harper is suffering from dementia. Maybe she is in her final hours of life. But her ability to connect with reality (using this interpretation) has disappeared, and in her desperation, she longs to connect with the idea of home.