You will only be able to get airside with your boarding pass, so presumably when the check in counters open for your flight is the earliest you can get it and proceed through security.
Once you take the train to your gate, you cannot go back to the main terminal. Your gate area where EK and all international airlines depart is very, very sparse. Especially late at night. I wouldn't take the train to that concourse until the last minute.
That being said, ICN has a lot of activities outside security, like a movie theater in the train station and a sauna in the basement on the main terminal (landside). There are also tons of cafes and restaurants on all floors landside.
Once you pass security and are airside, all of your duty free shopping is right there once you clear immigration. There are some restaurants in the main terminal, but honestly I think landside has a better selection.
All your lounges and free showers are going to be in the main terminal, 4th floor, after you clear security. Of course there are lounges in the concourse terminal where your flight departs, but they are not as big.