Seat reservation during Obon?

I will be arriving Tokyo on 8 Aug and will be travelling extensively by train. Do I require seat reservation for the following:
a) Narita Express from Airport to Shinagawa on 8 Aug?
b) Shinagawa to Shuzenji on 10 Aug?
c) Shuzenji to Narita Airport on 11 Aug?
Advice is greatly appreciated!

Narita Express from Airport to Shinagawa on 8 Aug?

Seat reservations are always required on the Narita Express. It is highly unlikely that the trains get booked out on that route on August 8.

Shinagawa to Shuzenji on 10 Aug?

The ride can be done entirely by unreserved trains (local trains and unreserved cars on shinkansen). Therefore, seat reservations are not required.

Shuzenji to Narita Airport on 11 Aug?

See above.

As for traffic volume, both on August 10 and 11 can be expected to be quite busy, but not extreme.

will travel from narita to Fukui be difficult from on Aug 13th and do trains run until what time?

You can look up routes and schedules on the following websites:
http://www.jorudan.co.jp/english/norikae/
http://grace.hyperdia.com/cgi-english/hyperWeb.cgi

The best route is by Tokaido Shinkansen and limited express train via Maibara (or Kyoto). The last connection leaves Tokyo at 18:50.

August 13 is one of the more busy days with respect to train travel out of Tokyo. You should try to make seat reservations as soon as possible. The morning seems busier than the afternoon

As of now, the direct hikari trains between Tokyo and Maibara in the morning (between 8 and 12) are already booked out or have only few vacancies, but all other trains still have vacancies. It might be easier to get seat reservations for nozomi trains to Kyoto, but the journey via Kyoto is slightly more expensive than via Maibara.

The trains from Maibara to Fukui are also very busy. About half of the trains are already booked out, especially in the morning and early afternoon.

The situation with trains from Kyoto to Fukui is similar, but there are more trains and still more vacancies.