fyi: Your airline (Korean Air) has no requirement for an onward flight, but your airline (Korean Air) has to bring you back free of charge if your entry is denied. That's why they ask for the return or onward flight.
It's about opening a bank account, not the Visa extension. As long you don't hold the longstay visa, you won't get an account, even with "ticking all boxes". After the job is done, there's no connection to the agent anymore.