Can planet smashing cause mutual annihilation?

I've found some good information on the Halley engines for Planetary Annihilation and I'm wanting to try it. However, I've not yet found a good Galactic War battle to use it on. Most of the time a moveable planetoid comes up in the war, it's in a two body system: the planet you start on and the planetoid you can rocket up for smashing.

In these systems, it seems like there's no safe place to retreat to while the planet ending annihilation is carried out. So how can I tell when a planet smash will totally annihilate the target vs. just crater it up? More broadly speaking, what happens if both commanders are annihilated at the same time? Do you win or lose the engagement? If you're playing hardcore, does your Galactic War end in defeat?