3. Why are other races not alternatives as well? It appears only humans are different in the other universe. Romulans, Cardiassians, and Klingons are unchanged.
Why would they be changed? Just because Human history took a darker turn in this timeline, there's no reason that would've affected those other societies, except insofar as humanity interacted with them.
Mirror Universe is a parallel universe where the characters are inverted, it's not just an alternative universe where one different event caused a series of other events and they diverged permanently.
Humans in MU are vastly different than the ones in "our" universe (read evil, brutal etc); that can't be the result of a different time line.