This is a thought-provoking post. But I find major flaws in your argument. You've pointed out in your post that not all "conspiracy theories" are nonsense.
So instead of lumping all the people who believe in "conspiracy theories" together, whether it's about the "flat earth" notion, the JFK assassination or 9/11, it might be more pertinent to discuss why some people think critically about these and other issues, while others prefer to unquestioningly believe what they're told by an "authoritive source" that they trust, whether that source is a TV news journalist, a government leader or a blogger writing about the shape of the earth.
The "conspiracy theorist" label is, as you say, a derisive one, and it's generally used to silence any questioning of the establishment narrative by lumping it together with - as you say:

someone else's wacko belief