How do I prep a, temporary, root login before editing sudoers?
No backdoor is required, in fact really nothing (not even a backup) is required to edit the sudoers file. You can't get locked out of your machine.

A backup (sudo cp /etc/sudoers /etc/sudoers.bak) is more convenient though, because you can simply restore it with a one-liner.

Reasons:


sudoers is not required to authenticate a user at log-in
sudoers is not required to activate the root user with Directory Utility - only a working admin account. Authentication is done with the local OD services.
If you have a working Internet connection or a working local Recovery Mode (aka recovery partition) you can always restore the default sudoers file either from an online resource (e.g. pastebin or github) or as mentioned earlier: more convenient from a local backup file.
If you have a second Mac and some fitting cable you can always start the "broken" Mac in Target Disk Mode and transfer a working good sudoers file from the second Mac.




Tested in a Mojave VM by: 


simply removing the sudoers file completely
editing sudoers with nano and commenting out the root and %admin line, changing ownership of sudoers to 501:staff and make it 777


Proof: