The problem is the unsubstantiated logical leap from "weather modification is possible" to "contrails behind commercial aircraft are secretly dispersing chemicals in order to control weather".

If I was an evil government figure trying to secretly control weather, there are better ways of doing that (e.g. drones, rockets) than orchestrating a conspiracy that requires hundreds of thousands of people to stay quiet for decades.

Yeah, that's true. Besides, it would be morally wrong!