Hey there @webcore --
The Glide navigation is aimed at a specific sort of design goal. Scrolling through larger sections of content in an easy to navigate manner. So for your first point (1.) Your sections are going to have to be bigger than just a few small items. The navigation needs a bigger section of content so it can know when / where to trigger the navigation item styling to indicate he current section. If the section is too small it will get confused. There's not a way around it. It is expecting substantial sections of content as that is what it is built for.
And for the section item, regarding the number of navigation items and some not being visible (2.) With fixed positioning navigation you're going to be limited with the number if items you can have. It is a downside to that style of navigation. Just like a horizontal navigation bar that is limited by the width of a page, the vertical, fixed-position navigation like that in Glide is limited to the height of the browser window. With the style navigation you have in Glide you need the navigation to be in a fixed position and not scroll with the rest of the content on the page. This means limiting the number of items you can insert into the vertical, fixed-position navigation.
These two things are just limitations of using this type element on a page.