Parent comment’s author display_name

So to do the same thing but for the parent comment this should do it. This is untested but does the same thing as your first case, but with the parent comment object instead of the current comment.

if( $comment->comment_parent ) {
    $parentComment = get_comment($comment->parent);
    $usermeta = get_userdata($parentComment->user_id);
    echo $usermeta->display_name ; 
}

Does that do it?