If you want to get to Disney on Day 1, taking the N'EX to Tokyo and Keiyo line to Maihama would work. You might find it simpler than some other routes that require multiple train changes.

With a flight landing at 16.50, you might reach Disneyland around 7.30pm or so. That would give you a couple of hours in the park (assuming the park is open that evening). You might be too tired to enjoy it, though.

Thanks. The horror stories about the Keiyo line transfer had put me off - is that still a viable option, even with bags? Better than going through Funabashi or Chiba?

Hmmm. I'd consider it doable, but it might feel a little overwhelming if this is your first ever visit. Whether you can handle your luggage, is something only you can answer...

You might be able to make it onto the bus to Maihama, which leaves T1 at 5.35pm, according to Yahoo! Did you see that? I don't think that's out of the question.

One alternative that might work is to use Keisei trains from the airport:
17.52 - Keisei Access Tokkyu train (Slow train - Not the Skyliner) leaves Terminal 1 (Can also catch from Terminal 2, 3 minutes later)
18.31 - Arrive at Higashi Matsudo Station
Walk over to the JR Station (walk quickly, now)
18.37 - Musashino Line train towards Tokyo
19.01 - Arrive Maihama

I think that walk, even with the moving walkways, really sucks.

Hyperdia suggests taking the JR Airport Narita train to Funabashi, switching to the local to Nishi Funabashi, and then taking the Musashino line from there.

You can also take the Airport Narita to Chiba and switch to the monorial to ChibaMinato, where you catch the Keiyo line. That particular monorail is a fun one to ride since it is suspended from the overhead.

However, this train only runs once an hour at that time. Keisei and a walk, as above, is more frequent.