Can you show us the blue print code you are trying?
But to answer the question, you can set the tabs up and then use pages sections in each one with the parent option set to the corresponding subsection you want to display.
You can set the pages you dont want to show as unlisted status, and set the pages section to show only listed pages. I hope I understood what you trying to do there.
Did you manually create those files and folders or use the panel? I ask because i wouldnt expect a default.txt file at the top level. Thats usually where site.txt is.
Really, I hate YAML. Of course the different bill sections have to have different names. Dang.
To answer your questions:
The directory tree I posted above resided under /content.
Not display the pages: Most of all, I do not want plain (which is an empty directory) to show up in any pages-section. Guess a collection would help here?
In a new tab, you can set up a pages section and set the parent of these pages section the parent of the children you want to show. So if you want to show the projectlist page and the CV page, set the parent to site.find('plain').
No, a page section is only a list of pages. When you click on a page in that list, you are redirected to that page where you can then edit the form. If you want to edit the page content inside the tab, you would have to develop a custom solution.
Or you put the fields for those pages into site.yml and store it there, and then your page content remains empty and you pull in the stuff for those pages from site.txt. It’s an option, but I wouldn’t do that.