Hidden navigation at Kosel [SOLVED]

Yes. You can hide pages (of any page type under any theme) by dragging the pages onto "Hidden pages" (just above "Trash") on the 'Pages' tab (and then clicking 'Save' and 'Publish').
When a page is hidden, there will be no link to the page within the site's navigation menu but the page will still exist within the site so you can use the page's URL as you wish (e.g. send it to someone, use it as a link in another web page, etc.).

You can hide an 'About' page by dragging and dropping the page name onto the 'Hidden pages' section on the 'Pages' tab (and then clicking 'Save' and 'Publish').

I'm not sure what you mean. Can you explain in greater detail what navigation icon you are referring to (please provide a link to your web page or a screenshot if possible)?
The Kosel and Maribo themes both have a top navigation menu and the Boma theme has a side navigation menu.
I'm not sure if you are referring to the social media icons in the page footer or a custom link in the page footer (or something else) but hiding a page from the navigation menu (by dragging and dropping the page name onto the 'Hidden pages' section) has no effect on any links in the page footer.