In regard of QT_QPA_PLATFORM=phantom I had the same error as in this thread. Playing with phantomjs on the CLI gave me a error and that I had to set that. So I tought, what if kibana has a similar problem under the hood and I added that. This cleared up the headless error. From there I played with the urls.
I will create for each kibana pair a xpack.reporting.index and remove the setting from /etc/default/kibana and see what happens..
Thanks for the advice.