Regarding the Qo'NoS atmosphere issue, I am not sure if it was shown in TNG or DS9, but in Star Trek Armada, Qo'NoS is a dusty brown planet. In Starfleet Command (set prior to the Praxis incident) it is a blue-green fairly earth-like planet. To me, that would say that while the atmosphere was poisoned and such, they found some way to stabilise it enough to maintain life on the planet (at the expense of having really awful weather I guess).
What's a little bad weather to the Might of the Klingon's?