Manjaro uses mhwd hence the /etc/X11/mhwd.d/nvidia.conf is what you are looking for to change instead of xorg.conf, but please pay attention on what you change in it, and you have to edit it with elevated privileges in order to be able to save it. You can copy your config changes in the Preview of the Nvidia X Server Settings and paste/replace them in the right place.
For .xinitrc make sure it ends like this
nvidia-settings --load-config-only
exec $(get_session)
and also make sure .nvidia-settings-rc is owned by your user.