It's a murder mystery. Answering everything in broad strokes, without being spoiler free, on the main page of the game would defeat the purpose of playing it in the first place.
If you're wanting a central theme for what Password is about, then "dealing with trauma and grief" is not a bad one. Naturally as the one making it I'd recommend playing it rather than reading a synopsis, especially while it's in development.
I don't understand your last question, but each of Dave's friends is a potential "route" to flavour the mystery but dating them isn't mandatory. As such there's no harem route where you date all of them at once. Nor can you 'date' any of the supporting cast either.