As I said in my earlier post about this being a valedictory novel--the last of the Watch books, I think all of the characters have developed in ways that I, for one, was very relieved to see. As to Sybil and the question of "language"--I think objecting to her "swearing" (shit, bitch, etc.),is, in my opinion, prudish and unrealistic.
Sybil has been developed by Pratchett, slowly but surely. She has begun to play a significant role certain by TFE. While some of her comments (eg. "You never answered one of my letters.") are apparent non-sequiturs, they are only so if one fails to recognize what Sybil is saying here. Serafina has (to Sibyl's face) lyingly claimed to be her old friend who is trying to help her find Sam. In fact, she has never been the friend she claimed to be--and Sybil is recognizing that in this comment.
Sybil is just beginning to find her self-confidence at that point, but she is in full possession of it when she takes over the negotiations with the Dwarf King and does a far better job than Sam could have done. In Thud!, while she in understandably frightened by the attempt of the dark dwarf to kill her, she rebounds magnificently, takes over the arrangements for the three of them to go after the dwarfs, and even manages to get the family portrait done.
Sybil is, in Snuff, at her finest--a complete woman who is capable of following the "tea" rituals of her class (and thus not offending them), but she is vehement in her anger at their stupidity. She has been raised by her grandfather (the one who swore so vehemently and who came out to the shearing with his tenants.) I doubt that there are any swear words that Sybil has not heard. Shit, poo, manure--they're all names for the same thing. And, as her grandfather told her, when she came across a big pile of it, she should use her foot to spread it around to help the grass grow. Sybil, by this book, has finally achieved her potential--a wife, mother, leader of social causes and unafraid of offending those who try to make her follow their outworn and stupid norms.
Q, I rather agree with you about the Summoning Dark. But, I think that Terry tries very hard to deal with what is, at one level, a plot necessity.
Sam has to be able to see in the cave (as he did in Thud!) and to understand and speak "Goblin". The SD left a mark on him, and left him sane and essentially untouched (unlike all others possessed by the SD). It is a bit of a stretch to see it functioning this way--but it is calling for a kind of vengeance for those who have no other protector than Sam. Thus, it continues to help him save the goblins by giving him the ability to see in the darkness of the storm, because without his help (and the ability given to him to see by the SD), they would all (humans & Goblins, & chickens) drowned in the flood of Old Treachery. And Sam, as Sybil points out to him early in the book, has trouble recognizing his own value, his importance in the world. He is, after all, the person who at least brought about the end of the Koom Valley conflict (whether or not he was involved in the negotiations). As Sybil says, if something happened to Sam the chanceries of the world would be in an uproar, the news would flash across the Discworld and it seems likely that Vetinari's position might become a little dangerous. No wonder the SD regards him as an equal.
Sam's battle to recognize the goblins (henceforth) as sentient beings has, as Vetinari says, has been won. But as Vetinari also acknowledges, his win is
"Because of a song, commander. Oh, and of course other efforts, but it was your wife who got most of the ambassadors to her little amusement which, I may say, Vimes, was eloquence personified. Though frankly, Vimes, I find myself shamed. One spends one's life scheming, negotiating, giving and taking and greasing such wheels as squeak, and in general doing one's best to stop this battered old world from exploding into pieces. And now, because of a piece of music, Vimes, a piece of music, some very powerful states have agreed to work together to heal the problems of another autonomous state and , almost as collateral, turn some animals into people at a stroke. ...Lady Sybil is worth a dozen diplomats. You are a lucky man, commander."