I haven't read every post on this thread to date, so forgive me if this is a repeat theory. But I'm wondering if LE has cleared every person living in the surrounding houses/apartments. From interviews done with a couple of neighbors, the house was notorious for parties, people coming and going all the time. Could the house/occupants have been an irritant to an unstable neighbor nearby who wanted to "shut down" the party house. Would explain why there's very little security footage in surrounding areas of cars/people acting suspiciously (outside of the white sedan which may or may not be suspicous). Perp walked over to house and then returned home and has been holed up since the incident. A reclusive type person who isn't social, doesn't leave home much. Would also explain the potential knowledge of the layout of the house and the occupants activities - most nights and this night in particular.
Just my speculation/potential theory.