Aside from flat Earth, I don’t think the other examples are actually all that easy to disprove. What, for example, would constitute a disproof of lizard people? Genetic testing of every human? Most conspiracy theories are poorly defined, and this provides substantial insulation from reason. A purported disproof might be countered by declaring “ah, but that’s not the real lizard people theory - you see, they have anti-genetic-testing technology”.

I think this is part of the trap that believers fall into: the idea that if you can’t disprove it, then it might be true, and if it might be true, then a fun game of connect-the-dots begins.

Of course, an infinite number of things might be true, so the actual conspiracy theories that people choose to believe in start to be driven by their personality and social network. That’s why we hear about lizard-people but not, say, the equally plausible echidna-people.

Yes (unless you mean "falsifiable" in a very strict logical sense).

Any theory that requires millions of people to systematically lie and hide evidence, with little to gain, across the whole world, and for many decades, is extremely unlikely.

I don’t know enough about it to say, really. That’s the thing where airplanes don’t have condensation trails, but whenever you see clouds behind one they are crop dusting chemicals on people? You’d think by now someone who works on airplanes would have noticed the chemical tanks/sprayers, or sampled where the contrails landed for unexpected chemicals.

Someone who believes in it with just a bit of money could surely fly up and sample the air inside of one of the trails, for example.