Would it be safe to run `curl https://nixos.org/nix/install | sh` (safe as in it won't screw with my existing installed packages), and if so, will that get me Nix 2.0? Or do I still have to wait for nixpkgs-unstable to rebuild?

The easiest way is probably to do following:

  git clone https://github.com/nixos/nixpkgs
  cd nixpkgs
  git checkout origin/nix-2.0
  nix-env -i $(nix-build --no-out-link . -A nix)

If you already have Nix installed, what you want to do is change the channel, and do an upgrade.

Change the channel to what? It's already on nixpkgs-unstable.

nix.package = pkgs.nixUnstable;

nixpkgs-unstable was updated 4 hours ago. Still doesn't have Nix 2.0.

This strikes me as very strange. Why was Nix 2.0 released without updating nixpkgs?