The train system is very easy to use. Yes, once the exchange order that you buy overseas is converted into the actual activated rail pass, you can make future reservations for the entire duration of the pass. Reservations are free, and can be done on the spot at the same time you activate the passes, and/or at any later time. Although the former national railway has been broken up into 6 separate companies, they operate seamlessly as far as the pass goes and reservations go. With your above itinerary, you'll be using the services of 3 of the 6 JR companies (JR East, Central, and West), but everything can be handled at the JR East office at NRT and you'll probably never notice.

Also, JR signage and recorded announcements are all bi-lingual in Japanese and English, so you should not have too much difficulty in navigating the system. The people at the airport ticket office will speak English. But, not all JR employees do, and I would not expect people at other ticket offices to do so. But if you show them your pass, and write down your destination and date and time of travel, they'll figure out what you need.

Also, when getting seat reservations, be sure to ask for the tickets in English. If you don't, then the station names on the tickets will be in Kanji only, which may be difficult for you to keep track of if you have a stack of them. My experience is that the airport ticket office normally does this without asking, but elsewhere, it often does NOT happen unless you ask, even if it's obvious that you don't speak Japanese, let alone read it.