Who cares if it is a single class? People shouldn't be taking shortcuts through others' leg room. It is no more acceptable to cut though the space in front of row 18 than it is to crawl over the people sitting in rows 19, 20, 21, etc.

I've only been in Y on a 777 once. It was a 77L (AC034 from YVR-YYZ). I was seated in 19A on the flight, and people in the middle section of row 18 seats didn't really appear to have any more leg room than at similar bulkheads on an A320. If it were a case where people in row 18 could fully stretch out their legs and not even touch the bulkhead (i.e. still several feet of space), a case could be made for use as a throughfare; IIRC this is NOT the case. I have no idea how much leg room there is on rows 77L row 30 or 77W rows 18, 30, or 50.

If you are having a medical emergency and need to get to the washroom, ask the person if you can infringe on their personal space (if it is obviously an emergency they might let you).

I can't think of any reason why someone would need to cross at row 18 on either aircraft as the lavs are located further back. The 77L has one crossing point in Y at the rear of the aircraft; the 77W has 2 crossing points in Y at the galley and the rear of the aircraft. These aircraft also have AVOD. If you are watching a movie, you can pause it at any time to go to the lav, and movies/TV shows don't all finish at the same time. It isn't like in the days of mainscreen entertainment when you would have a huge rush to use the lavs when the movie ended.