I just had a horrific thought...
What if X&E came straight home from the frat and watched a movie, as X told her dad, and then went to bed...
What if the killer attacked them right then, between that 12:00 call and 1:00 when the survivors came home and 1:58 when M&K came home, and waited in their room...
What if M&K figured they had just gone to bed when they got home...
What if something still seemed kind of "off' to them, but not enough to call 911, hence the series of calls...
LE might have recognized a difference in Time of Death between the victims in the two bedrooms early on. Since E&X's timeline is so mysterious, yet you would think there has to be a digital footprint, maybe LE is holding it close to the vest to protect the investigation.
Nah...