I'm 6'8" and feel your pain.

Neither of these seats will give you much sleep. The traffic in front of 31C is busy because of the restrooms, like you say. 18C, on the other hand, doesn't have as much traffic but is slightly mis-aligned so your legs are sticking in the aisle of the PE/Y transition.

I've sat in 31D for YVR-PVG on the 788, and it was bearable. It has great legroom. I'd recommend 31D or G if you can get them. Just hope for no babies.