I have 2x i7 4750 with 1080ti cards and I have not had any issues installing and getting piplay working.
It was fairly simple as long as there are no internal gpu that are enabled. Note igpu are usually only on laptops not desktops
You should not need to install piplay as admin, and if you do it will be promoting you each time you launch piplay for admin confirmation.
Get the latest download of piplay from pimax website and then if you have klite codex installed, uninstall it as piplay will install the compatible klite codec as part of its install.
As mentioned it will also install a specific direct x and some Java (this is needed for it to work)
I also have the aourus gtx 1080ti cards but I plug my 3 monitors into the 3 display ports and use the 4k on the back hdmi (never tried on the front one)
I use the latest nvidia drivers as well on the latest piplay version
After I stalling everything you then need to unplug and replug the USB of the pimax cable. Leave it out for 5sec and then plug back in.. The led on the pimax should be blue.. If it's green the USB needs to be unplugged and replugged in
I would put your PC back the way it was
1 primary gpu and no internal igpu enabled
Uninstall all you have installed with the piplay so far
Remove all software leftovers
Uninstall klite if it's installed
Install a clean version of piplay as a user not admin, if it prompts for admin credentials then enter them.
Make sure that all of the software installs, directx Java etc and a piplay icon should appear on your desktop - if it doesn't then you can get @Pimax-Support to use teamviewer to help with the installation.
Hoe this helps