What the fuck? 3K for a puppy mill dog? When they are TONS of dogs out there that need homes, plus supporting a horrible inhuman trade. Granted I bought my dog from a breeder, but a responsible one, who had papers for both parents, along with the mother, grandmother, on site, with the father down the road. I'm sorry that your MIL did something like this, you really should give her a piece of your mind.
On a side note french bulldogs can't deliver their puppies vaginally, I'd hate to see how the puppy mill breeder takes care of the bitch after she's had her puppies removed :/
She has this dog already in her possession at 5 weeks old? If so, and it ends up coming to you, I would get the pup some major, major socialization work in with other dogs as soon as the vet says you can. Even then, double or triple what other dogs need. I know your upset, and I don't blame you, but maybe this will end up being a great thing (in the long run) for you and the pup....
Ah that sucks. With regards to the worming issue - a lot of puppies are born with worms so I wouldn't worry too much about that. Probably had to be treated twice because although the worming treatment would have killed any worms on site, there are still those hidden ones that have yet to surface. It normally takes around 2-3 treatments to get rid of worms anyways. I hope that maybe this all turns out to great thing for you after all