NEX Train Airport to Shibuya

I want to buy a round trip ticket for the train to Shibuya and back.

It's saying it's reserved seating, but when I arrive at the airport, can I leave immediately or do I have to wait till a train/seat is available?

Same for the return trip, I have to catch a flight, do I need to reserve a specific time in advance or can I just go to the JR station and take the first train available?

Thanks,

You purchase your ticket on arrival, and can purchase your return when ever you like. In ten years of using the NEX i've never seen it even half full so you have no reason to worry.

Tickets are assigned by specific train, so you should go to the ticket counter and ask for the train that will be leaving next and purchase your ticket with reserved seating. That is the correct way to use the N'Ex.

If you have enough time in NRT,
you can also reserve return N'EX seat ticket(s) at the same time.
(validity period of round discount ticket matched?)
http://www.jreast.co.jp/e/pass/nex_round.html

Already been answered about N'EX seat booking conditions,
other transportation services (cheaper bus etc.) pushed rate of share not many passengers usual,
so it is hard to think that it will be full except for super busy season.(GW,Obon and around Year end.)