Almost all of the Economy Plus seats on the 777 are indeed on the wing. Even in United Business one needs to be in the first two rows behind the L2/R2 door to avoid most of the wing.
There are two versions of the International 777-200. One version has forward crew rest bunk and is used on longer flights to Asia. The aircraft on United's 777 service from Chicago to Seattle continues to Narita, but as far as routings go, Seattle-Narita is a shorter TransPac flight. For legroom seats 21 H and J on these aircraft are the best, considering also you'll be on a Westbound flight in the Northern Hemisphere in the morning you'll also have less sun on the right hand side of the aircraft.
The seatmap on Seat Guru shows an accurate depiction of the location of the wing. The last seats in the row are normal seats and may only be less desirable if you find the proximity to the galley bothersome.
Hope this helps.