Gallery Only Displaying One Thumbnail

Try this and verify that the attachments come back correctly when queried directly:

// helper function to return first regex match
function get_match( $regex, $content ) {
    preg_match($regex, $content, $matches);
    return $matches[1];
} 

// Extract the shortcode arguments from the $post
$shortcode_args = shortcode_parse_atts(get_match('/\[gallery\s(.*)\]/isU', $post->post_content));

// get the attachments specified in the "ids" shortcode argument
$attachments = get_posts(
    array(
        'include' => $shortcode_args["ids"], 
        'post_status' => 'inherit', 
        'post_type' => 'attachment', 
        'post_mime_type' => 'image', 
        'order' => 'menu_order ID', 
        'orderby' => 'post__in', //this forces the order to be based on the order of the "include" param
    )
);

echo 'gallery count:' . count( $attachments );

If that count is correct, then the gallery shortcode should be returning correctly. Let me know what that returns.