As I expected might happen, I got some guesses on the nature of the mirror universe characters which were much more interesting than the “official” answers. But here are the real ones anyway.
Mirror Poid is self-serving and competent, rather than helpful and incompetent.
Mirror Axl has his intelligence reversed, and Mirror Rusty is female instead of male. (Those were pretty obvious.)
Mirror Scatchi is low-intelligence animal creature in the position of a full crew member, the reverse of Scatchi’s situation. (Assuming “child” and “animal” to be interchangeable, which is fair, I think. This one was still pretty unclear, though, wasn’t it?)
Mirror Benjamin has a cursed life instead of a charmed one, and has become a humorless battle-scarred mercenary.
And Mirror Piper has a sociopathic lack of guilt rather than a neurotic overabundance of it. Although I like the guess I received better — that she kills parasitic worms and eats their brains.
(Extra: Since doing this story, I discovered that the show Lexx has a whole season with the same “twin desert and ocean planets” setting. Whoopsie! And I only intended to rip off Super Mario Galaxy.)