When you are planning to travel more than once to the same destination, then you need a second ticket. Example: Paris – Oporto – Barcelona – Oporto – Milan – Oporto – Paris (2nd ticket for the Oporto – Milan – Oporto route).
Yes, you can. If you want to get to your destination by going through a more distant award zone, the miles needed will be calculated as the sum of the award zones, based on the flight segments booked.
If you wish to return on a flight that departs from a different airport to the one where you landed, you can, even if the return flight airport is not located in the same award zone. The number of miles charged will be calculated as the sum of the award zones. Example: Flight from Lisbon to Madrid in economy class/Return flight from Copenhagen to Lisbon in economy class (20,000 miles).