Claiming Website

To fix customizer I’d suggest enabling debug mode. You can do that by adding the following code to wp-config.php

define( 'WP_DEBUG', true );
define( 'WP_DEBUG_LOG', true );

Once that is added try to load customizer again then look in your wp-content directory for a file named debug.log and see what errors were generated. This should help you to figure out what is causing the problems that stop customizer from loading.

As for the instagram feed. check out this plugin https://wordpress.org/plugins/instagram-feed/

Hope this helps. Once you find a solution you are happy with please tag the thread as resolved.