Unexpected chemical reaction: "Play to reduce a single Alien's strength to 0 until the end of the game round."We play you have to resolve all your moves before resolving any of their actions taking place. You shouldn't have action points left to bring it back this turn regardless. Otherwise you could move your army of aliens into one location, resolve the combat and continue moving onto a new location with the same group. Not allowed.
If the reduced Alien is destroyed (for example because of entering the "Electric Power Plant") and returned to the game (by reinforcement) in the same game round, is it still reduced (because it's the same figure) or is not reduced anymore (because it counts as "new" alien)?
Besides that point, it is a new alien so it isn't zero anymore.