Question: Carrie is seen crying after she realizes that Tommy is dead. I can only assume that she actually cared about him but she still believes he was good and didn't take part in the prank, right? And at the end does she actually care about Sue and forgive her?
Answer: In this version, Carrie never weeps for Tommy's death, and it's never indicated whether or not she was even aware that he died. And she never directly encounters Sue in this version, so we don't know her thoughts about her either. The film was likely trying to leave some things open-ended, so that audiences could interpret them as they wanted.
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: Since blood was dumped on him too she must have realized that he wasn't a part of it, so yes, she cared for him. In the book when Carrie touches Sue, they link minds and Carrie sees that Sue felt bad about teasing her and was trying to do something nice to atone, and had nothing to do with Chris' nasty prank. So yes it's safe to say she grieved for Tommy and forgave Sue.
Grumpy Scot