Before jumping into the main issue here, I looked into the 404 issue you had on another thread as well and that seems to be a problem with your web server configuration. Everything but
/wp-admin and the home page is showing a 404. This happens when your webserver doesn’t have the right configuration to for send incoming requests to the WordPress
index.php file. You will want to check your
htaccess rules to make sure they are correct on that server: https://wordpress.org/support/article/htaccess/
Now for the live preview error message. I’ve checked your site and it’s the exact issue I’m seeing on @al2code’s site as well (The preview could not open due to iframe response being incomplete)
Something changed on your server that is causing the HTML to change. On both sites, I’m seeing this output in the iframe response:
Are you aware of anything running outside of WordPress that could be rewriting HTML? If I do “view source” on both of your sites I see a similar code that starts like this:
<script data-ezscrex='false' I don’t recognize it and I’m not sure where it’s coming from. Do you know what this might be? Let me know, thanks!
Edit: to confirm regarding a question in your first post: Cornerstone is not required if you are using Pro.