Hi there,
As the trip from Narita to Hiroshima is around ¥20,000, and a 7 day JR Pass is ¥28,300, if you intend to travel to Hiroshima by rail I recommend you activate and use the JR Pass to get down to Hiroshima once you arrive at Narita. Furthermore, the difference between a 14 and 7 day pass is ¥16,800, so taking a 14 day pass here would also be an economical option, and gain you added flexibility for your trip.
Some things to bear in mind:
- The JR Pass can be used on all Hikari and Sakura class bullet trains, but not on the Nozomi trains that are inside your link. The Hikari and Sakura class bullets do require a change at Shin-Osaka en-route.
- To get to the bullet train terminal at Tokyo station I recommend you use the Narita Express train from Narita airport (covered by the JR Pass)
- The last Narita Express/Hikari bullet combination that will get you into Hiroshima on the 26th leaves Narita at 5:16pm. If you are unable to make this time, then the only other option is to either purchase a Nozomi class ticket (last train leaves Narita at 6:15pm), or possibly look into discount flights from Narita to Hiroshima. Depending on your inbound carrier, internal connecting flights can be had for a reasonable additional charge.
Hope this helps!