Otto, the Sheriff has said that the perp(s) were only in the house for a very short time. I believe the neighbour misread the time of the shots, it was likely 12:58 not 12:38, making the actual tome 12:51 when you adjust for the 7 minute clock discrepancy. This would make the first shots two minutes before the 911 call.
You might ask why would we doubt the neighbours testimony? Well the Sheriff has stated that this crime happened quickly, that it all went down in a couple of minutes. How would he know this? What type of timekeeping could he have that would allow him to discount the neighbour's testimony? The only possible answer would be that Denise's injuries were used to estimate the maximum amount of time between the second gunshot and the 911 call. Her injuries were obviously very serious, and the coroner would be able to give a pretty close estimation as to the maximum amount of time she could have maintained consciousness. In my opinion this estimate was a short amount of time, less than five minutes. So with that information LE can deduce that if the shots were fired at 12:31, there is no possible way that Denise could have called 911 at 12:53. The neighbour has to be mistaken.