Doubt that you can do online or kiosk check-in; fairly sure that is prohibited for flights to US by the TSA. Similarly cannot checkin at the Airport Express station, only at the airport. No idea how long the NW lines are; I use CX and it has never taken more than 5 minutes to do checkin (I suspect NW will be somewhat longer). Security lines are very efficient at HKG as is the exit immigration, that usually takes under 5 min and never seen it more than 10 minutes. I'd guess you are looking at 45 min for the airport express (30 min travel plus wait time) and 40 min at airport (30 min checkin and 10 min security; perhaps less if you are NW elite but I've never seen their operation at HKG -- its a large terminal and I stick to the CX aisle).