implementing this plugin on a wpengine hosted site(caching off)?

Hi

I’m having some issues with this plugin on WPENGINE hosting. It works great in wpengine’s staging environment(no cache whatsoever here), but in production there is cache on the landing page(they won’t remove it from the landing page no matter how you ask).I’ve not put any code in the cached landing page so that the WPSM plugin isn’t called from it.

I’ve written a plugin that provides a shortcode for each webpage. i.e. [startpage], [quotationpage], [resultspage], [orderpage] etc etc. I’ve used shortcodes so that only pages with caching turned off use the WPSM plugin. I’ve double checked no cache using curl + URL.
When a page loads, the shortcode calls it’s function which gets an instance of wp_session(‘$wp_session = WP_Session::get_instance();’) to read/write to it, builds some markup and returns it for the page to echo, nice and simple.

In staging this works fine, but in production, often the session will change to another user’s session! when you land on a different page you get the other user’s session data values.

As I said i’ve not put any code into the cached landing page so that the WPSM plugin isn’t called from it, but presumably caching is still working enough to interfere somehow, I’m wondering if this plugin could ever work on wpengine or do i need to move the site to another host?

I’d prefer to stay with them and work on a solution, but if there is no chance of ever getting it to work properly i’d love to know.

I’m running: WP 4.6.1, JoinsWP theme(css based version), WP Session Manager plugin 1.1.2.

i also get these messages in Staging and Production:
[Mon Oct 17 06:59:44.820797 2016] [:error] [pid 26986] [xxxx:44998] PHP Warning: Cannot modify header information – headers already sent by (output started at /nas/content/staging/mysite/wp-includes/functions.php:3598) in /nas/content/staging/mysite/wp-content/plugins/wp-session-manager/class-wp-session.php on line 124, referer: http://mysite.com/results/

( not a real url in the message above 😉 )

These may or may not be of help or a red herring?

Many Thanks