No offense to the OP, but I have taken several of these cruises. The first 2 times when I expressed my concern for missing part of illuminations, the captain laughed and said that was a great rumor. They do not leave in the middle of the show! He said they would never do that.
(Think about it - you're at WDW, just paid big bucks for the cruise, and the captain would love a tip! Do you really think he's going to scoot out a few minutes early?)
I have yet to read a post from someone who's cruise was over in the middle of the show because time was up. . . anyone?