Slightly modify these lines
$content = $page->post_content;
if ( ! $content ) // Check for empty page
continue;
$content = apply_filters( 'the_content', $content );
to
$content = $page->post_excerpt;
if ( ! $content ) // Check for empty excerpt content & fallback to full content
$content = $page->post_content;
if ( ! $content ) // Check for empty page
continue;
$content = apply_filters( 'the_excerpt', $content );
UPDATE:
for adding featured image use
<div class="image"><?php echo get_the_post_thumbnail($page->ID);?></div>