a) While Nick Park was still at the National Film and Television School. He had character sketches in his notebook prior to that and developed them at the school where Gromit went from being a cat to a dog with a mouth to a dog that didn't speak.
b) Paradoxically the specificity of W&G make them universal, and the themes and medium (plasticine) reach out to all people. Plus there is a worldwide curiosity about what is perceived as British quaintness.
c) Dunno I don't think about money 