I'm late to the party, just watched all the episodes.
I believe that she was a party girl which is normal for someone her age, she went off the ship on her own will, likely to get drugs with Yellow. Perhaps when she left the room she just did it to wander around and saw Yellow again.
No shoes, she may have taken different shoes, not the ones on the balcony. Now, for some Americans it is normal to go without shoes, especially around the beach. Not taking her wallet, no need it to wander around, then Yellow told her to come with to get drugs or she could have just taken cash from her wallet without taking all her cards.
Once in a lawless town with a drug dealer, they could have easily prevented her from returning. They would have seen her as a "high value asset" due to being a middle class American young woman. Mafia has a lot of power, especially in lawless towns.
I do believe the account of the Canadian that he saw her with Yellow, he didn't know about Yellow and he recognised both of them, many coincidences. To then, dedicate his free time to help build the website and track IP addressed.
Then, those pictures. It's definitely her.
So sad. If what that ex navy said it's true, he should have helped her. If he physio couldn't, at least put an anonymous report with the police.
Although, I hope she fell overboard over the other possibility of trafficking.