display wordpress user who published a pending post of another user

get_the_modified_author(); isn’t going to tell you who published the post, just who last edited it. You will need to capture you publisher yourself.

function post_published_notification( $ID, $post ) {
  $publisher = wp_get_current_user();
  update_post_meta($ID,'my_publisher',$publisher);

}
add_action( 'publish_post', 'post_published_notification', 10, 2 );

Then use get_post_meta($post_id,'my_publisher') to retrieve the data.

Of course there are a numerous ways to fine tune this. Do you want to only save the publisher the first time it is published, in those cases where that can occur? Etc.