The second up-train is likely to be very busy all week with Christmas shopping. There would definitely be an argument swapping the extra cars for the early train onto it for the week as the early train was very quiet yesterday morning.
You used to be able to reserve a seat for 3 euro each way plus booking fee but I don't know if this is still possible. This is the same as the add-on fee for a booking.
It's a waste of time booking a seat as if you leave it until the last minute, the chances are it will be occupied by some Anto who will knife you or some old dear who will guilt you into giving it up.
The shouldn't be allowed offer seat reservations without staff on the train to manage the boarding process.