here the idea for which I seek a solution:I want to have a system with no way to log as root. I want to manage the system (edit /etc/files, (un)install something, etc.) by physically unplug the disk and plug it on another computer, from where I want to manage with his own root account  the pluged disk.I have some (very incomplete) ideas to try to do that with a linux made from scratch. The problem a linux from scratch requires times, and I miss times... So, I would like to be able to do that with some usual distro, my target is Arch Linux, but if you are doing that with some other distro, I could try to port your solution.
how to avoid direct/network root login on the system and using some chroot to admin the system?
root;chroot;root filesystem
Linux-from-scratch is great but it takes a lot of time like you say.You can lock any account by the command passwd -l <user>. If you do this with root then be sure that you have a working sudo configuration.chroot is used to run applications in a sandbox. If it is hacked then the hacker cannot change the rest of your system. Managing a system should not be possible from within a chroot.