Change author slug and functionality

For the list of authors, the simplest solution is to create a page with the slug author, then create a page-author.php template where you’ll add your own code to list authors.

All views in WordPress are a post or list of posts- there’s no concept of an author list, or a list of terms in a taxonomy, those things need to be created manually.

For the author links, the fix is fairly simple, you can manually set the structure they follow to omit the front value that gets inserted by default:

function wpd_fix_author_structure(){
    global $wp_rewrite;
    $wp_rewrite->author_structure="author/%author%";
}
add_action( 'init', 'wpd_fix_author_structure' );

Don’t forget to flush rewrite rules after they change.