Hi derlokmerlo,
1. Reverse thrust can be easily described as a reverse in direction of the gas flow which in turn reverses the thrust component. By generating an extra-high drag component, the new "forward" thrust component helps the aircraft deceleration thus reducing its stopping distance.
As far as I know, the CO2 emissions are the same as in normal thrust. It has been proved that CO2/HC/CO emissions are significantly lower with the least number of engines running (new reduced engine taxi procedures are being implemented nowdays).
2. There're 2 taxiing phases: Taxi-in is the ground distance to the gate, covered by an aircraft after landing; Taxi-out is the ground distance covered by an aircraft from the gate to the runway.
3. APU (Auxiliary Power Unit) is a gas-turbine engine generator able to provide the necessary power to meet the demand of either the electrical, hydraulic and pneumatic systems.
What you mean for PCA phase ?! ...
afaik PCA is the Principal Component Analysis, that is nothing more than a mathematical procedure.
Hope that helps,
fredgrav