Yes you can change easily all the text/strings in the theme, if you also need to change the link of the button you can edit the files directly or use a child theme.
(2)That is actually a good idea, you can upload a ms word document and link it with the 'add link' button from the editor.
(3) Customizations are not officially supported by AppThemes but there is a Mods and Tutorials forum where you can find mods or get help from members of the community.Changing the header will require basic knowledge on php and css.