11/10 believable Bahamas theory
So, the idea is that Dan wanted to return to England while Phil would stay on his family’s vacation. But Dan for some reason accidentally bought a ticket to the Bahamas and flew there (that’s a very Dan thing to do). He’d posted the proof that the expiration date was 3 days so after that time he returned to Florida. When he got there, having nowhere to stay, the Lesters invited him to stay at their house so he decided to do it and go back with Phil.
It would make sense since Phil was meant to stay in Florida anyway so their videos have already been prerecorded and that’s also a reason why Dan had uploaded only one video (okay, let’s be real, there wouldn’t have been more video even if he had returned).
And that’s my totally believable prediction of how Dan got to spend his first vacation with Phil’s family.