It was reported to be cabin #8564.
Three witnesses correctly placed Allister Douglas with Amy despite the fact that, at the time of the alleged "sighting", they didn't know those two from any of the other thousands of people on the ship? If a "trade off" did actually occur, Amy would have had to have gone voluntarily as any struggle, particularly in an area as public as a disco, would be noticed.
So many incredibly implausible events would have had to occur for the trafficking theory to be even the slightest bit accurate--cooperation of numerous individuals, no one seeing or hearing anything on a crowded cruise ship, no one talking for nearly three decades, etc. If this case is ever solved (incredibly doubtful at this point), and the trafficking theory actually proved correct, would likely take the cake for the most implausible and unlikely missing persons case in true crime history. It's definitely an intriguing and interesting theory, but incredibly unlikely.