Why, when Oliver grew up in an orphanage in the slums, does he talk like the upper-class family he was born into? Are accents genetic?
This is probably reflective of the classist views that dominated 19th-century England, when it was assumed that breeding had a large effect on personality.
It's been a while since this troper's read the book, but The Movie of The Musical makes it pretty obvious that the Bumble and Widow Corney are upper-class, as are their dinner guests.
In the book they certainly aren't upper-class - they'd be provincial middle-class, possibly with ideas above their station. It's been a while since this troper's seen the film, but it's possible that any "upper-class" trappings are affectation.
This could be a bit of pandering to the audience - Dickens wrote the novel in part to show the awful poorhouse conditions created by the 1834 Poor Law, and the upper crust he was trying to persuade wasn't exactly going to be won over by a dirty little boy crowing, "Take yer bloody hands hoff me, ye fookin' wankers!" We likes us some well-spoken pretty boys, we does.
In the author's preface to the Third Edition, Dickens specifically mentions that he toned down the language to avoid readers being offended by that and missing the other points he was making about living in poverty.
One of the main reasons Dickens wrote the novel was to demonstrate the awful conditions the poor lived in, that it wasn't their fault, and that how you're raised has a much bigger effect on who you are. He creates little orphan Oliver, who owns nothing and who's mother wasn't married, but who is as pure as the driven snow. Great, he's doing very well (apart from the fact that Oliver's innocence is so exaggerated that he's inhuman, and he would be a complete Purity Sue if he was older). Only, then he goes and makes Oliver the child of a rich guy, completely undermining the whole 'personality is environmental' thing. Huh? That's just stupid . . .
Not only is Oliver the child of a semi-rich guy, the denouement has Oliver's father's will explicitly state that Oliver can only receive the inheritance if he never commits a crime, as Oliver's father wanted to prove that Oliver would inherit his mother's "gentle nature", just as Monks had his mother's hatefulness. So I can't even consider this a stupid accident.
You say yourself that it's one of the reasons Dickens wrote the story. He also needed to provide a story his audience would like. To appeal to readers, he needed a hero who suffers through hardship, overcomes it, and gets a happy ending. Oliver's inheritence is a quick way to get him out of the slums and reward his good behavior. The story is essentially a guided tour through London's underworld, forcing the reader to see what's going on without getting soiled themselves and ultimately arriving safe and sound back in polite society.
Well, except that Dickens's main point was to demonstrate the awful conditions that the poor lived in, full stop at that point. He never made any connection between upbringing and personality - there's just no correlation. There are good rich people (Brownlow), good poor people (Nancy, Charley Bates), bad rich people (Bumble, Corney), and bad poor people (Sikes.) If Dickens had any point regarding how upbringing affects personality, it's that there's no effect at all.
Bill Sikes is supposed to be a professional burglar, and a pretty good one. Now, pro burglars in those days did use children ("snakesmen," as they were called) to wriggle into places that adults couldn't, but why would he insist on using Oliver, who clearly isn't willing to go along with the thing? There were probably dozens upon dozens of ex-chimney-sweeps'-apprentices who'd have been honored to be noticed by a burglar of his stature, who also had the climbing skills he would have wanted in a snakesman.
Two reasons: 1) Oliver was the closest one at hand. 2) Charles Dickens just wanted to torture Oliver some more.
And a third, plot relevant reason: Fagin was being paid by Monks to corrupt the boy so he would never be a threat to inherit a fortune (which he would never get if he had a criminal record), so getting him to commit a crime was done with forethought. Unfortunately, Oliver is probably the last kid anyone with an ounce of sanity would use for the crime in question.
Monks is said to be one of the "chief members" of Fagin's gang, but he never seems to do anything for Fagin. How does this work again?
He could very well be the guy who fronts them money for operations. Even though he didn't get his inheritance, he does seem to have more than enough money to throw around to attempt to get his hands on a lot more, so it's plausible he was The Man Behind the Man of Fagin's gang in a financial sense.
Is Nancy sweet and motherly towards all Fagin's thieves, or was it just Oliver? She says that seeing him turns her against Fagin as if it's something new. Surely after thirteen years of seeing him recruiting other innocent little children on the street she would have gotten over it. Did she have fits over Fagin corrupting all his thieves? If so, were those all those fits Bill was talking about?
It's possible that she does throw a fit every time she sees a kid get corrupted, and does eventually get over each one only to throw another fit at the next one. Nancy's not the most rational of women. It's also possible she throws a minor fit every time, but only really went off on one with Oliver because he's basically a Purity Sue whereas Dodger and Charley were probably at least partly willing accomplices to the whole thing.