1. It is not necessary to display the tgas in the sidebar.
2. Yes, you can change the “pages” to anything you want. This is the template tag that displays the list: Template_Tags/wp_list_pages
(see the examples there)
3. see #2
Note. If you are using the widgets in your sidebar(s) – they overwrite everything that is hand coded in the sidebar.php file, in which case #2 will not have any effect.