I believe that the best answers for these are as follows:
The first of these is a metaphor. The poet is using the idea of being on a knife edge to show what it feels like to be in a dangerous situation where one could fall off (literally or figuratively) at any moment.
The second is alliteration. Alliteration is defined as using a bunch of words, more or less in a row, that begin with the same sound. In this case, three of the four words in a row all begin with an "s" sound.
The third is onomatopoeia. The word "thud" sounds like what it means. That is the definition of onomatopoeia.