I just don't see how that plugin will solve the problem I described. Perhaps I didn't describe it well.
When I view my list of categories in the dashboard it says that the Books And Story Apps category contains a bunch of posts.
But when I go to that category's archive page, it shows zero posts.
It does this because all of my posts that have the Books and Story Apps category attached to them are custom post types. WordPress supports custom post types, so the category sees that all these custom post types exist. But for some reason Archive pages, by default, only show posts of the "Post" post type (i.e., not custom post type). This behavior is entirely nonsensical, of course. I can't imagine that anybody would ever want
their blog to behave this way. But that's the current state of things.
The plugin you linked to, as near as I can tell, would allow me to make a page and attach categories to it. But that's just not what I'm trying to accomplish.
For what its worth, the main page had a similar issue. Only posts of the default "Post" post type were showing up on my main page. I had to add a function to my functions.php to add my custom post types to my front page loop. I assume that similar code could be added somewhere to accomplish the same thing with my archives, but I don't know PHP so I don't really know where to begin.
For now, I have come up with a workaround. Custom taxonomy archives contain posts of all types
that are attached to them. So I made new taxonomies to replace my categories. It's an annoying solution, and I'd still like to have my category archives working the way they should. But it's at least serving my purposes for now.
Please let me know if I misunderstood your original suggestion. But as near as I can tell, it would neither fix my original problem, nor provide a workaround. In any case, I appreciate your trying to help.