I'm not sure if I understand the first point. Doesn't Nix currently achieve the same thing feature-wise?

For example,

    spack install packageX %libA@2.0

would translate to

    nix-build -E 'with import <nixpkgs> {}; packageX.override { libA = libAv2_0; }'

For this to work, both Spack and Nix would have to package libA v2.0. If it isn't, the user would need to create their own. Assuming a different version of libA is already packaged in Nix, here's how it would look like without the "nix-build -E" part:

    let
      pkgs = import <nixpkgs> {};
      libAv2_0 = pkgs.libA.overrideAttrs (old: {
        src = pkgs.fetchzip {
          url = "...";
          sha256 = "...";
        };
      });
    in
    pkgs.packageX.override { libA = libAv2_0; }

I assume something similar would be required for Spack too.

So aside from Spack having a nicer shorthand syntax for customization, I don't get what Spack can do that Nix can't in terms of features. Or to be more specific, how a dependency resolver can eliminate human work.