Rose is attractive and fragrant with her perfume, but purely decorative, and has thorns. Thatcher is a builder of sorts, in his efforts as an educator to awaken his students to the wonders of the physical world; Greenwood could be an allusion to his naive, untested quality in personal relationships, and to his awakening to botanical science through Mary's companionship--although he thinks, when we first meet him, that it refers to his being "easily bent," and he does have a hard time standing up to his wife, her mother, and the headmaster Cutler. He sees his mother-in-law, Aurelia, as a cattleya orchid, a showy epiphyte, which grows by depending on other plants for physical support -- that seems appropriate.
Our 21st century family seems to have names reflecting literature, including ancient Greek, appropriate for a family with this background. Tig (for Antigone) is a character who resists authority as a moral imperative --as did her mythical namesake. Zeke, presumably for Ezekiel, seems at odds with his namesake, who prophesied that destruction and rebirth of Jerusalem. Zeke in this novel doesn't seem to have the insight that Tig does, although he does attempt to find a way to work with the system by starting a firm for socially responsible investment. Willa (named for Willa Cather) ultimately grasps Cather's insight about happiness. Dusty is short for Aldus, as in the novelist Aldous Huxley or possibly Aldus Manitius, a 15th century publisher and humanist--each of them a visionary literary figure. Dusty represents, clearly, the future. I'm not sure that these literary names are meant to reveal the characters,but they are apt.
I am struck by the question about their education or backgrounds; looking back, education is clearly important, but in itself, it doesn't correlate in the story with a character's willingness to think outside of the "cardboard box." Polly and Rose presumably have similar backgrounds, as sisters, and similar education, yet Polly is open to ideas, Rose is not. The maid Selma seems more interested in the world and open to it than Rose. Education and professional experience contribute to Thatcher's and Carruth's open mind, but clearly character is as much involved, as equally educated people like Cutler and Dr. Bowman cling to their conventional thinking and defend it with religious literalism, which confuses myth and metaphor with fact.
Zeke is well-educated by slower to embrace a new reality than his sister, whose insight "out of the box," and her courage in embracing the unexpected, is not from education but from her adventures and exposure to the wider world, including Cuban socialism. Similarly, Mary Treat's education was mostly self-education, and her hard experience with her estranged husband, and her correspondence and efforts to support herself in a patriarchal era, probably had more to do with her "beyond the box" thinking.
Willa, our protagonist whom we see actively struggling with this paradigm shift, is a liberal journalist coping with numerous family challenges, and unemployment -- she ultimately is able to stretch herself, but it is through being challenged by her daughter. I don't think Kingsolver offers a pat answer as to why some people can do what Willa did, or Thatcher, Mary, Polly, Tig, Carruth... but I think being exposed to challenge and perhaps hardship matters as much if not more than formal education. Both the head and the heart need to be educated, in other words, to gain the confidence to break out of the box.