I've noticed that several people have created mods that make all companions essential. However, I'm looking for a mod that makes them killable (non essential) once they have become your companion. It is my understanding that they work this way in hardcoe mode, but not in normal mode.

Is it possible to make this change in GECK, or have they hardcoded the killability of companions in the executable?

It's possible to make said change using the GECK. The "make essential" command is probably buried somewhere in the companion script and/or the hardcoe mode script. Deleting that if/endif block (provided it doesn't control anything else) should do it.

The only way to make them kill-able is to

 ActorRef.SetPlayerTeamMate 0

. The problem is that the death is intercepted by the engine before you can detect it in a script and turn off playerteammate. So for non-hardcoe the companions are essential - period. Baring some cumbersome work-around.

You can set a companion as essential in HC mode but in Non-HC mode the setessenial functions are ignored as long as the actor is a playerteammate.

Thanks for that extra bit of info. I saw in Veronica's script that her health and stats were reset after death upon check that variable. I was wondering where it got set.