Wow, you're really going to elaborate lengths of pedantry here, aren't you?
"Please define 'mainstream media' and no, I'm not satisfied with your definition."
Why is this important? You could make a list of 100 news sources and ask a random sample of 1,000 people to check Yes/No on "is this mainstream media?" You would get a very high level of agreement. It's not my responsibility to give you a definition you're happy with. This isn't a scientific debate.