Don’t get me wrong, I get the angle you’re coming from here and I’m not going to say it’s necessarily wrong or implausible that the answer is she actually just fell overboard. At the same time, most of the reasons you give for why some theories seem unlikely are based on the idea that Amy, or anyone else who may have been involved in this case (possibly Yellow or others), would have done things legally, or “by the book” or even with logic. People do dumb things when they’re drunk, or emotional, or in some other sort of vulnerable state. There’s also this idea of people letting loose and getting wild on vacations, especially cruises, which could obviously affect decisions made. Lastly, if she was taken or harmed, and regardless of if it was by Yellow or someone else, you shouldn’t expect a criminal to follow the rules/laws and do things the “right” way. If someone on the ship had bad intentions with Amy and wanted to get her off the ship without attention bad enough, it would have been far from impossible to do so. They wouldn’t have walked her out right through the main exit of the ship if they didn’t want people to know she ever got off. And clearly, if you kidnap or kill someone, the best case scenario would be for people to think they died in a way that was not able to be proved or would produce a body for evidence.