At this point, why would police investigating Myrtle’s death consider that case closed?

they considered the case close

Explanation:

because they didnt have enough proof

In the excerpt from the poem "Thanatopsis" by William Cullen Bryant, the meaning of the word "swain" is a young shepherd or country boy.
The narrator really works this image of our bodies swinging into the dirt. Here he lectures about how a country boy or the  "swain" – a pretty famous guy in aged nature poems) digs up that clod of dirt with his plow ("share") and walks ("treads") all over it. That’s just how low you’ll be after you've perished. Even the swains get to stomp on you.