This strengthens the argument by giving us a direct reason to believe that stories that cannot be well understood by readers are not likely to be covered in magazines. This in turn supports the author’s conclusion that interesting but difficult topics will not be covered.

This is irrelevant. The author isn’t making such sweeping claims about whether popular magazines do or don’t publish interesting stories, just claiming that some particular stories that would probably be interesting are not published.

This is irrelevant, since the author is only claiming that complex scientific issues would usually make for some of the most interesting stories to readers of popular science magazines. How common or uncommon those issues are doesn’t matter to the argument.

It doesn’t matter to the argument whether readers are able to assess their understanding of complex scientific issues, only whether they actually do or don’t understand those issues.

Whether or not readers find other sources to read about scientific issues is irrelevant, as the author is only making claims about what scientific issues are likely to be covered in popular magazines.