I forgot about the vacuum. Though one of the reasons being that in Alien Isolation they clearly have no problem with a vacuum, at least exposed for a couple of minutes. I know it's not the most logical idea, but I always liked seeing the alien survive for at least a short while in space while the humans are in their clunky spacesuits. Ads to the the alien's toughness, making their hard and creepy exoskeleton/skin a kind of a spacesuit of their own.
To be honest, even though I really liked the Rage War trilogy, the aliens with breathing apparatuses seemed kinda weird and silly to me personally. Though if this is the new norm, then I can't say I don't understand why. But lets not forget Alien Isolation...
I guess the best reconciliation is they could maybe survive a few minutes in a vacuum with no problem, but not for too long. They are mortal after all.
And yeah, the last third is definitely weaker, but over all I enjoyed the series. I definitely want more longer stories like this. And a continuation of this one as well. I would just like them to not change direction or the flow of the overall series when we get to the later issues.
I liked Fire and Stone quiet a lot, though Life and Death not so much... For me, the story was less engaging and the artwork didn't help. That's why Defiance and Dead Orbit are so refreshing.
All in all, can't wait for more.