You're in luck, I've just got off the ship on Jan 15, 2005. So, I can answer all the questions you need.
1) There are no chairs around the basketball area. But, there are two stairways on the two sides of the basketball court that you can sit on. (the court is slippery, becareful
2) I do believe that all rooms are the same size, though I am not sure. I stayed in Deck 4 with the window, and yeah, the room is small.
3) Sushi? well, I don't think they had any when I was there. There was smoked salmon that served for breakfast, and that's the closest to sushi that I've seen on the boat.
4) Don't worry about express disembarkment, the regular disembarkment was pretty speedy. I believe Deck 4 was the first to get off for all independent cruiser, and we got off around 9:30-9:45am. So, we had time to eat out last breakfast before we left.
BTW, I know this site has a few bad reviews toward NCL SEA, and I was a bit skeptical about this whole trip before I went, too. But, I had a great time and the trip definately worth the money. Be sure to eat at the restaurants, they offer great food and service. Le Bistro is great as well, but, the food wasn't that much better.