For 1., it's possible (easy even? I've never tried this in a cross-compiling situation) to build packages on one system and push them to another via SSH.

I suppose an upgrade would still result in two copies of everything, at least temporarily, but at least the target system doesn't need source code, compilers, etc.

https://nixos.org/manual/nix/unstable/command-ref/nix-copy-c...