In that particular example I have been checking programs that are free software as they are hosted on Github with free software licenses. I gave you hyperlinks as references, you could verify it yourself.
No, I (and really, it is not about you or me here -- it is about the casual user) cannot -- since the web browser runs the program before I can do so. Level of verification is never perfect, regardless of the type of software. How do I know that software delivered in Guix or Parabola GNU OS is free software? I do not know, I can just assume as developers claim to be so, and OS-es are endorsed by FSF. They don't only claim it, they also have policies that implement what things are added. Additionally, you can download the software before you execute it. You speak of verification, but there is none when it comes to pasting a URL into a web browser. Making it easy to run non-free software is not something that is useful. The reset of your message goes into tangets, and so I won't answer it. You also again confuse that the issue is webassembly, it is not. It is how it is used, which is like Javascript.