I don't think Xelayans are any more indestructible as other humanoid species, they're just comparatively stronger because they're adapted to and born in high-gravity, and so have "super strength" against materials and in environments adapted to lesser gravity. I did get something of a sinister vibe from Talla, with her "punch now, ask questions later" thing. I've already deleted the episode, but when she was strangling the doctor who was going to extract their tooth samples and Ed stopped her, she seemed loathe to let him go, and she said something like, "You're lucky this time." I think her propensity to violence is going to come up again. (I agree with the rest of your post 100%!)
Kelly and Bortus' genius plan was, "How can we stop these people whose prejudice believes us to be violent, homicidal criminals? I know! Let's be violent, homicidal criminals!" The astrological belief system doesn't really hold up. I can get behind that, through cultural conditioning, the other signs might be self-fulfilling prophecies. If you're indoctrinated into the global religion and are told all your life that you're going to be a great leader because of your star sign, get all the opportunities and education to track you along that career path, then you'll probably end up a great leader. But have no other signs committed violence or crime in their society, especially when it's still a capitalistic society and thus open to currency competition and wealth inequality, corporate greed, homelessness and poverty? Further still, the Giliacs in the concentration camps said that no one has ever tried to break out before, because they accept the dogma that they're inherently worthy of incarceration. They've passively accepted their lot in life, their history is 100% nonviolence, and yet the rest of society hasn't caught on that Giliacs aren't acting according to prescribed belief? With a track record like that, where are the skeptics and atheists in this society?