I think you have to tell Zach that Elena is nice in the club, if you don't she won't go in for the kiss.you just largely go for choices where you get Layla points instead of Elena and you won't even get the option in the hot tub to kiss Elena. Even if you have enough points with her, you can still decide against involving her, so it's entirely player driven. The train kissing scene is automatic I think, no points to be gained there.
Many other choices can knock Elena off, not helping her train, not staying with her and her brothers at the beach...
And as you said even after all that you can still chose to focus on Layla for a Layla exclusive scene in the bathtub, not even an option to tell Layla that you like Elena if you do that, you just skip straight to the bedroom.