Can't speak with any authority about the 747, perhaps someone else will dive in later.
Basically if you are off for the 11 days a month and you are commuting for 2 (might be 3 for the 747 fleet) then you are available for work for 18 days in a 31 day month. You will work most of those days with obviously your legal days/time off between trips.
I don't have an answer to your tax query, a Google search will throw up the Korean tax offices website and you can work through that.
At the interview you will be asked why you want to join Korean? What do you think of Korean and Korea? How do you feel your Sim ride went? There might be some technical questions nothing too heavy. They will ask you a few questions about yourself. Speak clearly and slowly, and you should be fine.