I spent a couple of days trying to reverse proxy three XI instances with Apache with the same results. I tries rewriting all sorts of things in URLs and content. No go.
I ended up installing Thruk as a front end to our Nagos XI instances. Had to install livestatus alongside XI on each of the monitoring servers. I then proxied Thruk instead. This doesn't give us everything XI bundles, but I am looking into proxying components separately and configuring them in Thruk. Thruk gives us the "one interface" view of multiple data centers I was hoping Nagios Fusion would be.
Also looking into incorporating check_mk now that it is installed for livestatus.
My opinion is don't waste too much time trying to proxy Nagios XI, although you could proxy the core install itself and the nagios mobile component on the xi server
omegle shagle instead.