The irony is that they think that Columbur intends to harm them. But infact, columber wants to help them by giving them a pearl.
While there are many different sound devices that can be used in poetry, such as near rhyme (sounds are similar but not exact), alliteration (same beginning sounds), this one is onomatopoeia. Onomatopoeia is using words to suggest a sound. This can also be words like whispering, sizzle, thump, clang, etc. Using onomatopoeia allows you to imagine what it would sound like if you were present (this is part of imagery).
It let's Janie have the freedom that she craves, yet she doesn't completely change herself she just lets her hair down.
A topic is like a title of a book and a Unit is like the chapters in the book.