Less than you think. A lot of misinformation is spread by people who pretend to believe it, but do know on some level it is bullshit.

Yes, but much is created by bad actors knowingly spreading misinformation so it will go viral. I do not fault those who do not know better but I fault those who do or should by the result of their position. All leaders and candidates have an obligation to check there information. All purported news agencies or pundits as well. There is a different burden depending on audience an reasonable affect.

That's a special case that we can discuss when necessary.

I think it's useful enough for practical purposes that we can clearly classify something as misinformation where the spreader knew what they were doing.

The people often do. But at the root are people who knowingly create misinformation and spread it.