is it plausible to argue that Rose of Sharon is, ultimately, the most siginificant character in the novel?

THere is certainly an argument for this. In creating the character of Rose of Sharon, Steinbeck relies heavily on stereotypes. We read that pregnancy has transformed the girl from a “hoyden”—a high-spirited and saucy girl—into a secretive and mysterious woman. Time and again, Steinbeck alludes to the girl’s silent self-containment and her impenetrable smile. This portrayal of pregnancy may initially seem to bespeak a romanticism out of keeping with Steinbeck’s characteristic realism. However, Steinbeck uses such seemingly trite details to prepare Rose of Sharon for the dramatic role she plays at the end of the novel. When she meets the starving man in the barn, she becomes saintly, otherworldly. Her capacity to sustain life, paired with her suffering and grief for her dead child, liken her to the Virgin Mother and suggest that there is hope to be found even in the bleakest of circumstances.