Golden Week 2016

Hi,

I'm going to be arriving in Tokyo at Haneda airport on May 3rd, staying in Tokyo for a few days and then I would like to take the train using JRP to Kyoto for a night on May 5th, then go to Yahata station the following day for a few hours and then return back to Tokyo on May 6th.

1. Am I able to book/try to make train reservations from Haneda airport?
2. Since I will be traveling during Golden Week, do you think that there is a chance I won't be able to book a train ticket once I arrive in Tokyo on May 3rd?? Will that be too late to make seat reservations? I anticipate leaving each morning around 10am or so.
3. If I buy the Green JRP, can I book / make a reservation for my train tickets now in the United States?
4. For the stations that I'm going to: Kyoto - Yahata, does the green JRP afford me that much more?
5. I read online that "Travel activity is anticipated to peak on April 29 and May 3 with people leaving the large urban centers and on May 1, 5 and 8 in the opposite direction." So maybe my traveling from Tokyo - Kyoto - Yahata is going the opposite way as most Japanese on vacation??

Thanks in advance for your help!