Mollii,
Just to clarify, yes, I am Japanese, and I know that it is as normal and abnormal in Japan as it is in most Western countries to do things alone. That is why they have this sort of slangish prefix in Japanese called "hitori-" and people are surprized every time someone says they do "hitori-yakiniku" or "hitori-karaoke" or what not. And really, I'm the only person among my friends that go to rock concerts alone.
lastmockingbird,
"assuming that 5250 yen is one-way, is it safe to assume that twice that would be the roundtrip?"
If you're taking the same route and same type of trains? Yes, of course. And I repeat that you need to ADD the express fares if you're taking express trains that allow you to be their within 2 hours.
"Also, I am not familiar with how to purchase ticket train in Japan. I plan to purchase, if I do decide to go, at the JR ticket booth at NRT terminal 2."
No problem, as long as the date of the ride is within a month ahead. But I see no reason to buy those tickets to the concert upon your arrival to the country. You can just purchase the ticket when you're riding the train at Akita to go to Sendai. Also, you shouldn't be reserving seats, as there is a risk of missing trains. Again, don't forget to present your student ID if you have one that is official in Japan.
"Upon purchasing, will I be given a ticket that applies only to a particular arrival time of the train?"
If you purchase a reserved seat, yes. You have the option to purchase tickets for the unreserved section that would be valid for any train of the route of that day.
By the way, JR East has an English iformation telephone line.
http://www.jreast.co.jp/e/customer_support/infoline.html"Also, at the NRT, can I already buy a train ticket going back to Akita from Sendai?"
Yes, but again, you have to specify a date of the ride.
I don't know anything about hotels, but JR Infoline can give you some advise, and you always have the option of having coffee at the 24 hour McDonald's Sendai-Higashi-guchi branch (the upper one on the map.)
http://www.mcdonalds.co.jp/shop/map/map.php?strcode=04520"sounds pathetic huh?"
No, that sounds very practical. In fact, that's what I do all the time. But I hate (?) to say that the train ticket is ALWAYS available. Even reserved seats are most likely to be available. I think it's more practical to say that you should give up if the "concert" ticket is not available. Or to make it easier on you, why not give up if you can't find someone to accompany you?