Since the dialogue is translated into English, the southern accent means that the dogs are speaking with the French equivalent of a "redneck" accent.
It seems strange that Madame Bonfamille never suspects Edgar. But she had no clue at all that Edgar even knew of her will. And even if the police suspected him (the crime is mentioned in an in-movie newspaper, so it seems reasonable that Madame called the police), there are no witnesses or evidence that link Edgar to the crime. Roquefort and the horse do know that Edgar is the culprit, but they can't exactly testify.
Based on her comments at the end of the film, even if she had known he knew about the will, she wouldn't have suspected Edgar, because Madame Bonfamille seems to have thought it obvious that, even if the cats inherited first in law, in practice this would probably mean Edgar would inherit everything on condition that he took care of the cats.
Why would Edgar think of getting the Madame's money in terms of dollars when French Francs would've been more appropriate as it was the currency at the time? Because dollars are worth more than Francs.
Marie flutters her eyelids when she's calling it "romantic" that O'Malley is allegedly a philanderer. Cats express affection by slowly blinking. Also, maybe the Gabble sisters assumed O'Malley was a philanderer because he's a cat — cats aren't monogamous.
The amount of sleeping pills Edgar put in the cats' milk would easily have been a lethal dose. Was he just putting them to sleep and then abandoning them? Or did he fully intend to kill them and was just disposing of the "bodies"?
For all the talk about Edgar being unwilling to kill the cats, a trip in a trunk to a faraway country would eventually have done just that. Even with sufficient air holes, the long time it would take to deliver it (this is 1910 after all) would have guaranteed that Duchess and the kids would have slowly starved to death inside. And that's exactly what Edgar's fate is unless he has the common sense to make lots of noise while he's still being carried off to the distribution depot. I'll take the classic fall off a tall building, thank you.
Yes, this is because Edgar is really, incredibly stupid. He'll probably get out of the trunk at the post office when he starts making noise and will be too embarrassed/scared to return to Madame's house.
And not to worry, since this is a Disney film, the cats probably DO have nine lives.
The fact that Edgar keeps a shelf with sleep medication right next to his stove. You have to wonder if there's been other instances of Edgar trying to poison someone.
Or he has problems sleeping.
The kittens' dad is unknown. Let's hope he was just a stray or neighbour's cat who knocked Duchess up instead of her being a widow, because if she's a widow, that means a cat died.
More than likely, the kittens' father was a pedigree tom who Madame Bonfamille set Duchess up with for breeding purposes. It would be the logical action of a wealthy doting cat owner.
Also, Madame Bonfamille is a kind cat lover more than willing to adopt Thomas on the grounds "that every household needs a male", as in she could see Duchess and Thomas being attached to each other. Either the kitten's dad died of natural causes, either and most likely Madame de Bonfamille set up Duchess with a pedigree tom from another affluent family.
