You'd probably want to first check that /etc/resolv.conf is a real file, first. SystemD has a nasty habit of replacing it with a symlink. That prevents "chattr" trick from working.
2. it is not a nasty habit, but a configuration. If you do not want it, you can configure systemd-resolved not to do this.
3. it is for a very good reasons, and there were written pages and pages about the issues, what those are; so when you understand why, you won't reconfigure (2) anyway.