The first of the series of national holidays known as Golden Week has been April 29th (the Showa Emperor's birthday) since at least 1948 as far as I can tell. The first peak travel time this year will be April 26th - the Friday before April 29th.

May 1st is kind of the eye of the storm - It should be a calm day between peak travel dates, so I think that you are actually "safe." Nonetheless, Nozomi trains will be reserved seating only. I think that you will be able to walk up and reserve a seat on the day, but if you prefer to reserve your Shinkansen seat in advance, I suggest that you opt for a train that departs from Shinagawa no earlier than 10:40. That assumes that you'll clear immigration and customs by 09:00 and get on the next Narita Express (departing 09:12 from T1 and 09:20 from T2), which reaches Shinagawa at 10:25. Of course, you could give yourself more cushion to allow for possible late arrival etc. In that case, you will be able to get the ticket changed on the day or kill time at a cafe in the station.