Narita Express (N`EX). Return Trip Discount Tickets. (Are the times flexible)?

**Narita Express (N`EX). Return Trip Discount Tickets. (Are the times flexible)?**

It seems that N`EX discounted tickets are open to anyone with a foreign passport. I plan to grab a set at Shinagawa, and will return the same day.

When you book the tickets, are those times firm? The website seems to suggest everything is reserved, but you CAN get on a train without a reserve ticket, you just need to defer to reserved ticket holders when it comes to seating.

I am waiting for someone at the airport and if customs/immigration takes too long I may miss the scheduled train.

Also, I take it a foreign passport is all you need to get these tickets at the JR counter in Shinagawa?