Sort/list/arrange comments by comment author in un-nested/no thread comments

If you mean the comments in comments_template() then you can use the new comments_template_query_args filter that was added with ticket #34442 in version 4.5:

/**
 * Filters the arguments used to query comments in comments_template().
 *
 * @since 4.5.0
 *
 * @see WP_Comment_Query::__construct()
 *
 * @param array $comment_args {
 *     Array of WP_Comment_Query arguments.
 *
 *     @type string|array $orderby                   Field(s) to order by.
 *     @type string       $order                     Order of results. Accepts 'ASC' or 'DESC'.
 *     @type string       $status                    Comment status.
 *     @type array        $include_unapproved        Array of IDs or email addresses whose unapproved comments
 *                                                   will be included in results.
 *     @type int          $post_id                   ID of the post.
 *     @type bool         $no_found_rows             Whether to refrain from querying for found rows.
 *     @type bool         $update_comment_meta_cache Whether to prime cache for comment meta.
 *     @type bool|string  $hierarchical              Whether to query for comments hierarchically.
 *     @type int          $offset                    Comment offset.
 *     @type int          $number                    Number of comments to fetch.
 * }
 */
$comment_args = apply_filters( 'comments_template_query_args', $comment_args );
$comment_query = new WP_Comment_Query( $comment_args );

Example

Here’s an example where we target non-threaded comments:

/**
 * Order non-threaded comments by comment author, in the (main) comments template
 */
add_filter( 'comments_template_query_args', function( $comment_args )
{
    // Ignore threaded comments
    if(    isset( $comment_args['hierarchical'] ) 
        && 'threaded' === $comment_args['hierarchical'] 
    )
        return $comment_args;

    // Our modifications
    $comment_args['orderby'] = 'comment_author';        
    $comment_args['order']   = 'ASC';

    return $comment_args;
} );

Example

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