You can only select seats after payment. They will only open the literal back half of the plane for advanced booking. At web check in time, the entire plane opens up for booking. They do charge for exit rows and bulkhead seats and the first few rows of economy cabin. Anywhere from 30-100,000W depending on flight.

Also, yes 50 mins is doable. Korean Air exists to transfer passengers from Asia to the the rest of the world haha. So, they do these connections all the time. If your flight is really delayed and super tight, someone will literally be standing at the door with your name to personally rush escort you.