Confused with Hong Kong Airport Express (AE)

we will be holidaying in hk from 10 jan to 17 jan 2015. our flight back home is about 8pm and we need to check out by 12noon. to avoid carrying the luggages, we are thinking of in-town check-in at HK station.

1. we will be travelling by cab to city when we arrive in hk on 10 jan
2. we will be travelling around hk by mtr using the octopus card
3. for our last day in hk, we plan to in-town check-in first then continue with last minute shopping. we will take cab to the airport.
3. for in-town check-in at hong kong station on the last day, we will need to use the octopus card to cross the gantry for the check-in? how much does it cost?
4. after checking-in, can we leave the station to do our last minute shopping?
5. after our last minute shopping, can we take cab to the airport for our flights?
6. is it compulsory to take the AE train to the airport if we choose to in-town check-in?