That's like saying that logging in as an ordinary user "requires root" because otherwise /home/ordinary_user wouldn't exist. This is silly.

In any event, the nix store doesn't need to be in "/nix/store" anymore: https://nixos.wiki/wiki/Nix_Installation_Guide#nix_2.0.27s_n...

If you're willing to recompile the nix interpreter and sacrifice cached binaries you don't even need namespaces or ptrace. Just

  ./configure --with-store-dir=xyz

when you build the nix interpreter.