You CAN book LNER seats online…..link below, for ticket number just enter 0 if the site doesn’t take your cover # and if you want a specific seat you can then edit your seat after you have made the reservation.   https://www.lner.co.uk/travel-information/make-a-reservation/

Except the ride on London Undergr from Padd to Euston station must be paid =not included. This should be done by bank/crd card on enter/leave gates.

Oh-and yes, you can also come back same way-or other if you fancy to see more of this island in the Northsea. SEats on the LNER train Lon-YOrk can be REServed for free -only on a counter in the UK.

Sure you can. No reservations needed, just enter the train on your pass and hop on the train. On a travel day, you can use as many trains as you like.