Go back and take a good listen to the lyrics of "Cruella De Vil". Roger refers to her as "evil", a "vampire bat", an "inhuman beast", calls upon her to be "locked up and never released", and opines that "the world was such a wholesome place until" she came into it. While none of this is exactly inaccurate, he's still defaming a specific, identifiable living person (after all, neither "Cruella" nor "De Vil" are common names). Isn't he setting himself up for legal retribution from an outraged Cruella, who will forever be associated with a hit song that maliciously insults her from beginning to end?
While its possible that Cruella could try to sue him, her reputation is probably already pretty bad - she was a suspect in the police investigation for the theft of the puppies, which was all over the news, she more often than not Drives Like Crazy, which makes her a clear danger to pedestrians and other drivers, and shes pretty bad at hiding her vile personality when things dont go exactly as she wants.
She could just not feel insulted - if you're literally named Cruella De Vil, you might be a Card-Carrying Villain anyway.
Didn't Pongo get some of the black washed off earlier? Why is he completely black at the end?
Consider the scene where Cruella barges into the Radcliffe's flat just minutes after the puppies were born. It wasn't just good timing on Cruella's part—we can see later on that Jasper and Horace were spying on their flat the whole time. Cruella was outright stalking the Radcliffe's for their pups.
Because it's easier for young viewers to keep track of six humans, who can wear different outfits, hairstyles, and so on, than a similar number of dogs who differ solely by spot placement and how pudgy they are?