Irrelevant. Misinformation is information that is known to be false by the spreader. They can not make the argument that they honestly thought it to be true.
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.
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.
Since you seem to be splitting hairs: if I think the Earth is flat and I tell people this, my belief makes no difference to whether it is misinformation or not.
My belief means that it is a falsehood rather than a lie.