How should I get the original comment while processing the reply

Question

The comment/reply ID is a value actually sent by the form. You are able to retrieve it via $_GET:

// Default is "no reply" eq. 0
$id = 0;
// Handle replies
if ( isset( $_GET['replytocom'] ) )
{
    $id = filter_var(
        $_GET['replytocom'],
        FILTER_VALIDATE_INT,
        # or:
        # FILTER_SANITIZE_NUMBER_INT
        array(
            'options' => array(
                'min_range' => 1,
            ),
            # @link http://www.php.net/manual/en/filter.filters.flags.php
            # 'flags'   => '',
        )
    );
    if ( FALSE === $id )
        break;

    $id = absint( $_GET['replytocom'] )
}

Then you have access to the reply-$id.