You shouldn't have a problem making it onto the 18:52 N'EX to Shinagawa at the very worst case. I think my personal record is arrival on JL3 @ 2:40pm, deplane and through immigration in 5 minutes (Japanese passport), bags / customs in 10 minutes, go to train station and buy N'EX ticket @ JR ticket machine in 10 minutes, and board the 3:18pm N'EX to Tokyo.

The big factor for you will be the line at the JR ticket office. I use my Japanese credit card to buy tickets at the JR ticket machine (and I'm ineligible for Japan Rail Pass anyway) but those lines can be truly horrendous sometimes. I have seen "60 minute wait" posted there in the past.

Even if your immigration/customs + ticket exchange takes 2 hours total, you should still be safe for 18:52 provided the flight arrives on time. And as someone said the N'EX -> Shinkansen transfer at Shinagawa is very easy and fast. Make sure you change there and not Tokyo station!