Re b): Yeah but, like, colloquially "root" means "a process in the init user namespace with all UIDs set to 0 and a full capability mask".

Re a): If you are root in that sense (and haven't been blanket-denied the ability to use capabilities like CAP_SYS_ADMIN by an LSM), and not subject to a strict seccomp policy, then you cannot really in general be securely constrained with LSMs.

The kernel essentially treats CAP_SYS_ADMIN in the init userns as the catch-all for "you have been granted the ability to administer and access anything on the system", for anything that doesn't have a more specific permission and isn't access-controlled by UID. And if you can, like, call swapon() on an arbitrary file to make the kernel swap memory from the whole system into that file of your choice, LSM-enforced security boundaries probably don't work all that well anymore.