I think by "plot but non-relationship" she means something that is more or less independent from which girls you are dating, things related to the MCs dad, Emily with John, Ethan stuff...Depends on what she means by "relationship developments". It could mean few (if any) lewd scenes, but Elena/Layla fallout (if you reject Layla for Elena) could fall under plot development more than relationship development as if both girls are mad at you there won't be any way to develop the relationship with Elena, but there'd still be plot development showing how she reacts to it.
In a previous post Nyx did say that most LI will appear through phone calls and things like that.
I'd say the girl with highest chance of lewds is Brooke since we are going to be all day at the restaurant.