1)What's the difference between "all registered users" and "registered" in group access?
It's a custom usergroup that doesn't actually exist in Joomla. It's purely PHP and done through GJs API.
2)Is there a possibility to hide the link "back to overview"? It's disturbing when I have a menu for one category. But I don't want users to guide from this category to other categories.
Yes, edit the category_panes.php template file and remove it.