Question: At the end of the movie, Charles and Carrie decide to spend the rest of their lives together without being married. If Charles loves Carrie so much and wants to spend the rest of his life with her, why doesn't he just marry her?
Answer: I don't recall if it was determined whether or not they eventually married. The end showed them together with their baby and implied that they had married by then.
He specifically asked her not to be his wife.
Question: If Carrie loves Hamish so much, why does she still hang out and sleep with Charles?
Answer: Carrie was immediately attracted to Charles and gave in to her passion; she even said she thought they had a missed opportunity. Most likely, she never truly loved Hamish, was infatuated with his status and wealth, and probably felt it was too late to back out. Carrie really didn't know what type of man Hamish was.
Question: Who is Scarlett? She appears to live with Charles, but not share a bed with him; IMDb refers to her as Charles' sister, but her, er, style of talking is so out of kilter with Charles and indeed all the others that it seems unlikely that she's related.
Answer: You'll find the answer in this short deleted scene... https://youtu.be/ucNrqcqM3v0.
Answer: According to a deleted scene, she was found passed out drunk under the kitchen table after a party that Charles hosted and never got around to leaving. She's now his lodger, presumably paying rent and a share of the bills. https://youtu.be/ucNrqcqM3v0.
Question: Why wouldn't Charles' brother be his best man? Or is that not traditional in England the way it is in America? It's not mandatory but is certainly expected, unless the brothers are not close (which Charles and his brother are) or the groom's father serves as best man.
Answer: A groom can choose whoever he wants as best man. As you say yourself, it's by no means mandatory for it to be the groom's brother, even if they are close. In this particular case, it's possible that David felt that he ought not to be the best man, due to the difficulty that he'd have in making a speech, but, really, Charles was under no obligation to choose him.
Question: When Charles is being dropped off at the Boatman, why do his friends start making siren sounds?
Answer: Carrie had just gone through a divorce, and Charles had left a woman at the altar. They were both kind of soured on the idea of marriage.
Brian Katcher