So to clarify the JR will also allow us to use the Narita-Tokyo train as well (assuming that we activate it upon arrival) or if we wait a few days and its activated for the return Tokyo to Narita? I think its called the N'Ex?

Adding two more questions regarding purchase of JR?

Is there and advance purchase requirement for me to buy the JR? ie., X number of days prior to arrival?
What do I need in order to purchase the ticket? I'm guessing A ticket into (and out of) Japan?

I've read the JR site and the best that I can make out of it, is that I'd have to buy it at JAL at TBIT LAX (which is where I'm departing out of on an AA Award ticket - I hope that doesn't matter)