*As per eNotes policy, I had to limit your posted question to one main question.
When deconstructing any piece of writing for tone, focusing on the author or poet's word choice is always a good place to start. Pastan's poem "Ethics" makes use of contrasting imagery as well as diction to establish a several shifts in tone throughout the poem.
Pastan's first season focuses on the season of fall; the connotation behind fall suggests a slowing down of the natural order of things and the natural order of the aging process. This image is sharply contrasted by the youthful innocence of the students who do not really seem to appreciate the depth or complexity of their teacher's question.
The students' lackadaisical approach to the ethics question creates an apathetic mood which the Pastan seems neither to condemn or approve; her tone is not really nostalgic, because she does not seem to associate many overly warm memories with that particular classroom experience, especially when her teacher reprimanded her for "eschew[ing] the burdens of responsibility" (15-16). The imagery of the children "sitting restless in hard chairs" suggests a bleak quality to the memory, as if the memory itself is as uncomfortable and unforgiving as the backs of those chairs. With these images in mind, Pastan's tone seems more reflective than anything else.
By the ninth line of "Ethics," the speaker's thoughts turn inward, focusing on the image of a "this fall in a real musem [...] before a real Rembrandt" (17-18). Mentioning "this fall," the speaker indicates a shift to the present, and her tone darkens as she considers the equally dark colors of the painting. The imagery of the "browns of the earth" summons a feeling of power and authority; the speaker notes how they "burn through the canvas" (21-23). The speaker's connection to the painting and her admiration for the powerful quality of the colors suggests a tone of admiration, and then later, appreciation for the wisdom that comes with age.