Question: Why were the enforcers called sandmen?
Answer: In popular American lore, the Sandman is responsible for putting sand over your eyes to make you go to sleep. In this context the "sleep" is a euphemism for death, and the enforcers are responsible for making sure that everyone dies on time.
Question: When Long let Dwan fall into the water, we see that the water she plunged into is pretty deep. How deep was the waterfall when she fell in?
Answer: He probably knew it wasn't deep. It's his island; he knew every inch of it. Besides, it's a pool, not an ocean. It most likely had a small underground tunnel which spilled out somewhere on the island.
Answer: He starts to genuinely like her, hence his disgusted reaction at the sick practical joke played on her.
Manky