WHY would anyone just switch apartments? Diane would have a reason to do so when she had Camilla killed, but why would the other girl agree to it? Wouldn´t she suspect that something was wrong?

WHY would this girl not tell the police where Diane lives? The girl says to Diane: `Come on Diane, it has been 3 weeks` and `Those detectives came by looking for you.`Since she says `those` we must assume that Diane knows about them AND that they have come by no. 12 before. And this ´before` must have been (over) 3 weeks. So are we to believe that the police couldn´t find Diane in at least 3 weeks in such an important high profile murder case?

I think, their switching apartments because Diane wanted to have numer 7 in apartment number. 7 is her lucky number.