How can I display formatted content using the_content filter?

Question

I am using the_content filter to append comment summaries to posts on a blog page. That part of my code works. I am having a problem with the display of pages where they are not formatted and shortcodes are not rendered. What am I missing, or doing wrong?

function display_comment_summaries() {
$p = get_post();
$output = '';
$output .= $p->post_content;

if ( is_front_page() && is_home() ) {
  // Default homepage
  return;
} elseif ( is_front_page() ) {
    // static homepage
    goto display_output;
} elseif ( is_home() ) {
  // blog page
  goto display_comments;
} else {
    //everything else
    goto display_output;
}

display_comments:
$num_comments = 5;
if (( $p->comment_count ) > 0) {
    $output .= '<p style="font-weight: 700; margin-top: 10px;">Latest Comments (' . $p->comment_count . ')</p>';
    $output .= '<ul style="list-style-type: none; margin-bottom: 10px; padding-left: 0;">';
    $comments = get_comments( array ( 'post_id' => $p->ID ) );
    if (($p->comment_count) < $num_comments) {
        $num_comments = $p->comment_count;
    }
    $i = 0;
    foreach ( $comments as $c ) { 
        if ($i < $num_comments) {
            $day = ltrim( substr( $c->comment_date, 8, 2 ), '0' );
            $year = substr( $c->comment_date, 0, 4 );
            $month_name = date( "F", strtotime( substr( $c->comment_date, 0, 10 ) ) );
            $date = $month_name . ' ' . $day . ', ' . $year;
            $output .= '<li>' . $c->comment_author . ' on ' . $date . '</li>';
            $i++;
        }
    }
    $output .= '</ul>';

    $output .= '<p><a href="' . get_permalink( $p->ID ) . '#comments' . '">' 
        . 'Read Comments</a></p>';
}
display_output:
echo $output;

}