So is book one just going to be a separate story to the others,
They are going to be different stories. I believe Nine said they will be different characters too so they have 5 episodes to get it on.
I guess they don't feel like "relationships" at all to me,
Probably because they aren't the focus.
These are based on books Nine wrote and in those the Kylie was asexual and not interested in sex with anyone so it's been adapted for the adult audience.
The main focus is on what happened to Lisa and the lore of the world, relationships are there for those interested but there isn't enough time to make a grand romance.
It also says that there will be different ways to solve the mystery as you play but everything I've seen so far plays out in one way. There's no two ways about anything so far, aside from how much info you want to share with someone. Aside from finding the message on the back of the sticky note. I'm only up to the lunch with Mara but so far it's been completely linear.
Investigation mode is a thing where you hunt for clues based on what has been said. It's been made easier with clues now having a highlight but solving it won't come into play until it's time.
Like has been said, these are only short so having grand in depth branching paths just isn't possible. Whether you engage in the sexual side or not is up to you but solving the murder will be up to what you learn while playing.
You seem to be looking for a longer, slow burn romance story with routes to follow but this is a short murder mystery with the plot as the focus.
I'm not going to be one of those "if you don't like it don't play it" idiots because it's never black and white like that. Some people will always dislike something but enjoy it overall however things regarding the relationships aren't going to change so that side of the game is going to be a take it or leave it.
Nothing we say is going to make to the game longer in order to stetch out the relationships but, to speak for myself, I hate slow burn games. I have never seen the point in constant teasing and prolonging what we know is going to happen and anyone who says it's "realistic" has obviously never been on the dating scene. I'm grateful I get to see pussy and eat pussy. Hate it when i'm made to wait for my meal.