So I just wanted to give an update on my experience. I'm departing on my outbound 29 days from now, but my return leg isn't for 44 days. I just got off the phone with Korean Air and was effectively able to choose all of my seats for all 4 legs (even those further out than 30 days). I'm sure if you have a much longer duration there might be an issue but it seems as long as your first flight is within the 30 day window you should be good to get them all.