I haven't been on the Emerald, but I think the policies are the same as those of the Grand and Caribbean Princess which we have sailed.
I assume you mean dining room attire on the casual nights. If so, that means pants & shirt for men, skirt, pants or capris & top for women (similar to what you would wear to a nice restaurant at home, meaning no shorts or beach attire). According to the latest Princess dress code, jeans are permitted as long as they are not ripped, frayed or otherwise worn.
I'm not sure about late seating and showtimes, as we always choose anytime dining and work in a meal around the show schedule. I imagine that you would be able to catch the early show, as they usually run 2 performances, and still make it in time for your later seating.
Have a great cruise!
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08/02/2012 ~ Ocean Princess, Ireland & Scotland~~ 07/29/2013 ~ Royal Princess ~ Grand Mediterranean