I think Nirvana were the pop group of grunge, they made records to appealed to more than just punk, alternative and metal fans who were already listening to bands like Mudhoney, Tad, Melvins etc. There was something a bit more catchy to their music where some of the other bands were playing music that wasn't as instantly likable and needed a few listens.
The whole first grunge band discussion can go the same way as the first punk band discussion and there is no right answer. Grunge was something that evolved out of alternative rock and hardcore.