This can never work, since myEnv and pythonEnv are two separate environments that cannot overlap. You only add myEnv to nixpkgs, and then don’t do anything with it, hence only the modules from pythonEnv are available.
The docs say as much:
One limitation of this is that you can only have 1 Python env installed globally, since they conflict on the python to load out of your PATH.
If you get a conflict or prefer to keep the setup clean, you can have nix-env atomically uninstall all other imperatively installed packages and replace your profile with just myEnv by using the --replace flag.
There may be a workaround if you dig into the guts of this, maybe you can merge two environments somehow, but it will be very hard to update the python3.withPackages contents.
I’m afraid I have no idea why nix-env can’t access your overlay, the documentation clearly says that should be possible, perhaps it’s a bug.