Normally, Zimmer & Armitage will only become nonessential if you do the quest a certain way. This mod makes it so they're never essential.

The only problem I know of is that people will usually attack you or run away if you kill Zimmer or Armitage. If you have a mod to fast travel from interiors, you should be able to fast travel right after killing them before anyone becomes hostile. If you're using Quo Vagis & you've progressed far enough, you can quickly escape after killing them by asking a nearby woman to pose for a magazine so you'll automatically teleport away. Otherwise, you can type "coc megatoncommonhouse" in the console to teleport away or maybe use a stealth boy to sneak away.