Apologies if this has been answered already; I could not find it.
If a ship is destroyed before it has taken its turn in the initiative order, does it still return fire?
You're right. The rule definition itself does not explicitly say that destroyed ships stop participating in combat. But if you look in the combat example, you'll see this:
Iain rolls 6 and 6 for the Plasma Missiles in his remaining Interceptor.
So it is clear that only remaining ships fire. Destroyed ships do not fire and likewise do not heal after combat.