When you Archive an Article, the content is put into a state which removes it from your site as published content. The Article is still available from within the Control Panel and can be retrieved for...

To completely remove an Article, select the Articles that you want to delete and move them to the Trash. Next, open the Article Trash in the Content Menu and select the Articles you want to delete. After...

You indeed can change the Menu Item's Type to whatever you want, even after they have been created. If, for instance, you want to change the Blog Section of a Menu link, go to the Control Panel->Menus...