Well, it's not wrong that Ruka appears again because she still, technically, Kazuya's girlfriend (yeah Trial, but trial still continue if nobody tell that it's expired).
And if we want to show Mami again, I don't have any idea on how that's happen. She literally don't have any connection at all, except she know the secret of rental things.
And for fun info, now the neighbour girl also know about that rental things.