I think the first thing we have to do is to not trust a single thing that comes out of JB's or MP's mouths. If we go to the core, all we have is a dead young woman found among other dead young women. It is highly likely that their deaths are related. In other words SG was murdered.
Given that, then, we have these options:
1. SG ran for her life, for good reason, tried to save herself by calling the police and getting help from neighbors. While she would have been safe if she had just sat down on the neighbors porch, she was in such a state that she could not and her instinct was to flee.
2. SG ran for her life, but not because she had good reason to but rather because she was confused/high. She accidentally ended up at the hands of the LISK who was unrelated to JB and MP.
I certainly think 1 is by far and away the most likely option.
What confuses things is that the killer(s) made a mistake - having killed many girls before, they thought they had it down - but somewhere somehow they made a mistake - SG got loose and she was very close to bringing them down.
They could not afford that she would survive - and they could not afford to carry her elsewhere - since they knew about the 911 call. Therefore, they just killed her on the spot, knowing how easy it is for a call girl to go missing and how unlikely it would be for the police to truly look for her.