Problem: Broken /etc/sudoers file
Solution: Boot into a live environment and fix it from there.
This obviously requires either physical access, but it almost always works, doesn’t care about what init system you have, and does not care about whether or not you have a root password.
The general approach is relatively simple:
1)Prepare a live CD (or other boot media) that is compatible with the OS you are using (mostly, this just comes down to supporting the particular combination of storage drivers and filesystems needed to mount the root filesystem).
2) Boot the affected system using that boot media.
3) Mount the root filesystem of the affected system somewhere.
4) Edit and save the file.
5) Reboot.
This kind of fix works because it completely sidesteps the access controls of the affected system, letting you do pretty much whatever you want. This level of unrestricted access to the system is part of why physical security is so important.