Unfortunately, there is no current way to install the nVidia drivers on Unraid 6.8.3 except for the nVidia Kernel Builder container from @ich777. IMHO, there's no problems with installing 6.9.0-beta33+, and then utilizing the official method of getting the nVidia drivers onto your system (I've been running the beta series on all my servers for months now), but it's all up to your own personal comfort level.
Yes. The normal upgrade procedure will automatically upgrade the OS and revert the changes the plugin made to the boot files.
If you really want to revert back to the stock 6.8.3, then it's easiest to simply download the zip file from https://s3.amazonaws.com/dnld.lime-technology.com/stable/unRAIDServer-6.8.3-x86_64.zip, and then overwrite the bz* files on the flash drive with the ones in the zip (10 files total)