Having watched enough "unsolved mystery" documentaries, they often leave out or don't address critical things to enhance the "mystery."
Let me give you a few examples from this...
1. Who let the witness girls back into their cabin and what is their testimony about the time?
2. The only and I mean only evidence that Amy and Yellow didn't both go back to their cabins and not leave again is the testimony of those 2 girls.
3. And this is probably the most baffling and unexplored element from the NETFLIX doc, not a single other passenger or crew member saw Amy, Yellow, OR the girls. That's pretty hard to believe. A cruise ship at 5:30 AM isn't hopping, but there is activity. Crew is up preparing for docking. Housekeeping staff is getting ready, maybe cleaning public areas... dance club? Kitchen/Galley staff is preparing for breakfast. Security walking rounds. Early rising passengers having coffee (that's what her Dad though she might've done), a morning walk/jog, getting up to take sunrise photos... or even getting ready for early excursions (my cruise in '99 had me on a motor boat at 7am skipping over waves to those pyramids in Mexico).
^This is really hard to reconcile, if you're going with any of the kidnap/murder theories. You've also got to believe that with the ship only docked for probably 8 hours and the crew on alert and having performed that all-hands search, she was still smuggled off the ship in front of the eyes of everyone watching. I know it all fits a Hollywood movie scenario, but it sounds wildly risky and about 1000 different ways such a plan could fail.
If there was a single bit of proof that she left that cabin, I'd feel differently about this. I think she fell/jumped and the Dad heard something and woke.