Just to clear this up...
The 911 call came in at 12:53am
Someone on the other end of the call hung up, and the 911 operator tried calling the phone back as well as the landline at the house. No one answered those calls.
Police were dispatched at 12:56am
Police arrived on scene at 1:00am
Here is the call log:
http://media.graytvinc.com/documents/barron+county+call+log.pdf
It makes a heck of a lot of sense to me that the shots were actually at 12:51am (which would be 12:58 by the neighbors clock) and that James was killed at that time. The perp then rushes into the home to find the other occupants. There may be a struggle of some kind, or Denise/Jayme may be hiding. One of them manages to make the 911 call at 12:53am after finding a hiding place or briefly getting away from the killer. The perp then finds them and hangs up the phone. Kills Denise. (Perp may have already wounded her when she was trying to get away.) Perp puts a gun to Jayme's head and forces her out to the car to drive away at some point before the officers arrive at 1:00am.