In a normal apache front-ended zimbra installation, you only have one instance of apache that you configure.
When you install zimbra as normal, it sets up its own jetty web server which listens on port 80 and/or 443.
If you want to front-end with apache, you first move the zimbra jetty ports to non-standard ports - maybe 7080 and 7443 - or whateveer you choose. Make sure you can log into zimbra through the web client on the new ports.
Then you configure your apache instance and proxy requests back to jetty running on the non-standard ports.
From your description, it sounds like you have installed 2 apache web servers in addition to zimbra - which would not be correct.
PHP and php-mbstring, php-curl, proxying, etc all need to be configured to run on the front end apache server.
My best advice to you ...
Why don't you follow the advice I linked in my previous post, and on the wiki, to get it working first with a simple setup Apache front-ending zimbra. And, once you get it working then try to separate your components, and hardening as required.