I took a look on the walkthrough and the easiest way to have all the scenes without compromising too much the story is:
>Having the "Threesome" enabled on the Helloween party.
>Sleeping with Melody on the 0.20 update
>On the 0.21 update, saying "No Thanks" when Mel offers to smoke.
>Join them on the swim
>Saying "You Should be Cautious" or "I wouldn't mind seeing more".
>Saying "look at antiques".
This will make the safest way to have all the story elements without having too much collateral damage (unless the developers want to bring something related to "smoke" on the future). The rest of the choices basically doesn't matter, but there's a big chance that if you don't look properly, especially with Leah, the final score of the girls would be small and that will affect the future scenes.
After all, in the way that the game is treating right now is a big date with four girls and in the past updates, when something like that happened before, a big sex scene happened later.