Alexis is easy to avoid romance. Don't visit her on the plane and don't hang out with her at the bar. Even if you accidentally started romancing her there are LOTS of ways in the first few episodes to stop it, the easiest being tell all details to Zach at the bar. Further in, when you meet Layla at the Pizza restaurant tell her all details should also end it. There are a number of others as well...
For Layla, just avoid a few critical moments and manage the points you gain, and she'll never try to romance you. If you show interest in Val, she even offers twice to help set you up with her.
You do not need to be on Alexis' path to romance Val unless you are going for the Secret path. I've got 4 Val playthroughs, and on all of them, none of the LIs have tried to romance MC.