People do walk around cruise ships without shoes on. I've been on two and I wouldn't do that, but people definitely do.
Also, everyone saying people in the room would wake up if she moved around - it totally depends on the people. If they're a few nights into their cruise (and the weird sleeping arrangements) then they would have got used to the sounds of people moving around at night and slept through it. I don't think the parents said they heard the "kids" come back from the bar.
Despite that, I still think she went overboard from the balcony. If she had leaned back on the rail, maybe her foot on the door opened it a bit, caught her off guard and upset her balance. If the hand and foot print bit is true. If not, she leaned over to look at something or be sick and fell. I think suicide seems unlikely but people get all sorts of ideas when they are hammered.
But either way, she could not have gone ashore to get drugs, they weren't in dock. How would that have been arranged? No mobile phones. If someone had said "meet me on the poop deck at 6am", why? And why go back to her room at all that case, with the risk of family hearing and asking where she was going?
Yellow was a creep, but I can't see any evidence he had anything to do with it, he just chatted up anyone and she was one of very few younger women on the boat. His daughter was idiotic.