I'm searching for similar solution. How may I shut the system's xorg auto reset on reboot. Maybe after that editing the xorg will remain permanent and the display attached wont be needed...
In my case gpumanager was rewriting xorg.conf. You can check the logs: /var/log/gpu-manager.log
I solved by disabling gpumanager at startup adding nogpumanager in grub loader.
See
https://askubuntu.com/a/732004 for details