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.
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.
Since cats can't exactly spend or manage money themselves, while it may have belonged to Duchess and her kittens in writing, the money would have almost certainly been Edgar's in function as long as he kept taking care of them (and cats aren't exactly high maintenance animals). Hell, at that point he could probably just hire his own butler to take care of them for him while he enjoyed the rest of the estate. Hence, there was no reason for him to dispose of the cats.