JR Kanto Area Pass

I will go Tokyo on 14 Feb 2015 and stay Karuizawa from 15-19 Feb2015, then return Tokyo on 19 Feb and go Yokohama on 20 Feb.

on 14 Feb, I will take NEX to Tokyo
on 15 Feb, I will take Shinkansen to Karuizawa
on 19 FEb, I will take Shinkansen back to Tokyo
on 20 Feb, JR to Yokohama, day trip
on 21 Feb, I will take NEX from tokyo to Narita.

Question1:
1) Can I buy Shinkansen tickets and JR Kanto Area pass on 14 Feb 2015, and validate the tickets later? ie. JR Kanto area pass to be used on 19-21 Feb. Can I reserve the seats in advance?
2) Does JR Kanto Area pass cover all the transportation on 19-21 Feb (the shinkansen from Karuizawa to Tokyo, JR to Yokohama and NEX from tokyo to Narita)?
2) Is getting JR pass much expensive than getting individual tickets in my itinerary(NEX narita to Tokyo, Shinkanseen Tokyo to Karuizawa, JR Kanto Area Pass)