Macos – initialize the environment when the shell loads

You could try a bash function to pass the arguments before the ampersand:

np ()
{
    /cygdrive/c/Program\ Files/Notepad++/notepad++.exe $* &
}

BUT the bigger issue may be the whole idea of not waiting until Notepad++ exits. Shell commands which use $EDITOR typically are designed to wait until the editor sends back an indication that the editing has completed. This usually means that the editor has exited (e.g. with vi or nano).

Emacs has a way to set your EDITOR to emacsclient and then when you're done editing, you hit a magical keystroke (C-x #) to indicate that editing is complete. If Notepad++ had something similar for cygwin users, I can see how this would work.

Another alternative is to use plain, boring, simple Notepad as your EDITOR and reserve Notepad++ for heavy use (I got the idea from this blog post)

Shell invocation is a bit of a complicated thing. The bash and dash man pages has INVOCATION sections about this.

In summary they says (there is more detail in the man page, you should read it):

When bash is                   | it reads
-------------------------------|----------
login shell                    | /etc/profile and then the first of ~/.bash_profile, ~/.bash_login or ~/.profile that exists.
                               |
interactive non-login shell    | /etc/bash.bashrc then ~/.bashrc
                               |
non-interactive shell          | The contents of $BASH_ENV (if it exists)
                               |
interactive (as "sh")          | The contents of $ENV (if it exists)

-

When dash is                   | it reads
-------------------------------|---------
login shell                    | /etc/profile then .profile
                               |
interactive shell              | The contents of ENV (it it exists, can be set in .profile as well as in initial environment)

I don't know about other shells offhand as I never use any of them. Your best bet might be to set a couple environment variables to point at the common location script and manually source that (when appropriate) in the couple of cases that doesn't cover.