I don’t have any recent experience (post pandemic) but the main factor will be how many other international flights are arriving at the same time (a quick look at the website showed there are 3 other flight from Korea arriving about the same time as yours, so it may take some time for you to clear everything). My experience tend to range from 20 min out of the door to 2 hours+. It’s really YMMV.

As for train tickets, not sure what you meant by ‘they’ (airlines, JR, the government?). Just like you booked 2 separate airline tickets, nobody is responsible for you missing the connection. You should really make provisions for delay in going thru immigration when you made your bookings. But I believed if you missed your train which you have a reserved seat, you can just take the next trains within the same day with unserved seats.