I think times vary by season. I was there in Oct. last year, but I forgot what time...I want to say Noon.
All I know is, no matter what time of year you go, make reservations as early as possible! Also, show up early at the restaurant, I'd say at least 15 minutes. We had ressies for the 1st seating (and this is Oct. 2001, just a few weeks after 9-11), and the place was PACKED! We got there about 5-10 minutes early, and they did not even open the doors to the restaurant until the exact opening time. I THINK the list they had was based on when ressies were made. When we arrived, they put a number on our PS card and the hostess was putting them in order of number, not necessarily who showed up first. I only made mine a couple days ahead and we were not close to the water. Also, when we were seated, it's not like they were filling up the tables close to the water first, etc. Everyone was being seated in different areas of the restaurant.
I'm not sure if there's more food at dinner, but it is more expensive. I think the dinner menu is more extensive. Also, I think with dinner you might get a soup or salad included, but don't quote me on that. Lunch portions are pretty big though, if that's a concern.
I would check
www.mouseplanet.com for a menu, but they don't update regularly, so just use it as a guideline.