Common mode feedback for current mirrors
1) The currents of Q7/Q8 and Q9/Q10 are set by the bias voltages. It is very difficult to get this bias voltage exactly right without a feedback network, so the theoretical collector currents would differ. This discrepancy in current results in wild shifts in output voltage as a result of the Early effect (i.e. R_out in the small-signal model of each BJT).  

2) The DC bias itself is susceptible.

3) This portion of the question isn't that clear. We want to regulate the common-mode voltage because it is part of the desired specification of a differential amplifier: as the outputs swing up and down with a differential input, they swing around a well-regulated constant common-mode voltage. If we regulated Q7 and Q8 using only the voltage at vO1, we wouldn't get a fully differential amplifier; instead we would get something along the lines of a differential-to-single-ended amplifier.