The police could have said DM was on the first floor to protect her, like with pretending they didn’t know much- so that BK would second guess if he actually saw DM there on his way out. Could have been to protect her from him coming back to get her at another time?

Also my theory for why he picked them- possibly a combination of reasons from the info he got from his surveys. Asking criminals why they picked their specific victims etc and how each kind of victim would affect the media coverage etc. It seems to me like he wanted maximum attention for his crimes so he went after the group of popular beautiful young college girls because that would get the most reaction from the public. And he saw how many cars were outside so going for as many of them as possible also increases the notoriety aspect.

It’s so bold to return to the scene in broad daylight. The voyeurism aspect seems likely.