Hello! I have some ideas, but since I do not have your system/OS, I don't have an answer...
1) Yes, LibreELEC's system directories are essentially read-only. One way around this would be to make an exact copy/backup of your memory card and then edit that copy on another system (e.g. "Windows" via a memory-card reader) or a "live" linux CD or via a USB/memory-card reader mounted to the RPi4. Then reboot using this newly edited memory-card.
2) I looked at the "BOOTIN" script. I don't see the absolute necessity that the script be in "/bin" per se. It is not needed at boot time. It just needs to be in your PATH env (or current directory via "./") and executable. I.e., you run the script, it modifies the noobs.conf file as indicated (for Rasp. or Kodi/LibreELEC boot pointers) and then you reboot the RPi.
3) Do you actually have NOOBS installed? I ask since the script is modifying the noobs.conf file which is only going to be processed if you have NOOBS as the base installer for your LibreELEC, Raspbian and Lakka [?].