.... And then things with Layla were improving and it was a valid excuse to get out of it clean... In my defense MC is presented as a player or at least I always saw him as a player which is why:
1) I am deeply annoyed by how Elena's route works. Elena is a selfless and serious person with little time for herself, I am convinced that her relationship should be based more on not having time for the protagonist's bullshit and his prosciutto behavior
2) I continue to be convinced that Val is more of a test to see if MC continues to behave like a player than an LI. When Val enters the scene we are on the road to telling Layla that we are dating or have dated Lexi, rekindled the relationship with Nikki/Cara and started to create a micro intimacy with Erynn; And eventually you should ruin these relationships to run after Val.
I'm probably wrong about this but I still believe it.
At the very beggining it's easy to be a playa.
By the time we get the revelation that Alexis and Layla are sisters, i had already banged Alexis a few times and had a very flirty relationship with Layla.
The game presses you to chose your favorite Monroe or face the consequences, the one you chose completely alters the rest of the game.
Chosing Alexis changes the interactions with Erynn and Iris, opens Valeria as a possibility, and Elena becomes completely unreachable.
Chosing Layla, blocks Val completely but frees Erynn and Iris to act without concern, and Elena is now a possibility (in the loosest of terms), maybe as a throuple, maybe taking Layla's place(i also absolutely hate this path).
Gabi is a "freelancer" and can be combined with either sister(or any other girl)... but much more interesting if you are with Layla.
As for Val, i don't think she's simply a time bomb meant to detonate other relationships.
She does come in late, by the time the most likely thing is that players already have something cooking(like you said).
But i think her main purpose is to... keep the school interesting.
I'll explain:
If you chose Alexis, Layla is now just a friend and so is Elena, this would make the whole time you spend in school just friendly chit chat... i don't think many guys are interested in that.
In comes Valeria, she gives you a new option to flirt and have some action in school after you shut down Laylena by staying with Alexis.
She keeps the school interesting as a location on a Alexis run.
I think the fact that she's completely incompatible with a Layla save, simply stays a friend by default, lends some credence to my theory.