You should definitely voice your opinion about the conspiracy theory NOT being true, and then maybe try to gracefully chime in about her swearing, etc. Maybe tell her its not a good idea to portray herself in that manner if she wants "results" and to be taken seriously in whatever she is so passionate about.
Then, I would start to do less and less with her, but still keep get togethers ( for the kids ) and maybe if you can figure out the right amount of time to spend with her and not be annoyed, you will be happy in the friendship again.
As far as the majority not agreeing with her and to let it go, I think thats a comfort zone that you have, and it doesnt concern her. If she doesnt care what the majority thinks, thats something you either respect, or you dont agree with and thats that. If you dont agree with how passionate she is about whatever "cause" that the majority isnt in agreement with, then maybe it is best for you to part ways with the exception of playdates. You cant ask someone to change their mind just because you dont agree with it.