I've never bothered with the trains between HND and NRT, so I don't know if it possible to have enough space to work on your laptop on those trains. There is not a lot of room on the bus either, but if you get the front seats, you should have enough room to work.
The bus usually runs just fine and you should have plenty of time since it is the middle of the day (not peak rush hour). I've taken the bus several times from HND to NRT and have never had a reservation before arriving at HND. It is very convenient. Just remember to keep your passport with you (don't put it your luggage that goes under the bus) as the bus stops just outside NRT and they check everyone's passport before the bus proceeds into the airport.
Others might have different experiences that can chime in here.