Given the now available calls, here's my theory on Shannan:
For starters, I did/do believe that her case IS related to GB4. I also did/do NOT believe she died at the hand of another.
I believe her evening began like the others, serving as the entertainment for a group of folks - several of them being OB residents, the rest being a select group of individuals who may be of local significance. I also believe her evening would have gone on like the others, with her ending up the victim of a homicide. Instead, I believe she did frantically scramble from THAT potential situation into the marsh, already in a VERY physiologically excited state, under the influence of some substance, and collapsed and eventually expired from a medical emergency. Could be exhaustion, heart attack, overdose, stroke, or a combination. Her belongings being found elsewhere suggest to me that she may have fallen into the marsh for a minute. If you don't know what this is like, think quicksand. Beyond difficult to pull yourself out of, and would actually be likely to slide out of your pants to get free. Her location 130 feet from a roadway, especially when compared to the others, leads me to the conclusion of her traversing the marsh until she becomes trapped a second time and/or collapses.
My theory is she arrives at the house and begins to party. I do believe there are others there and this has a "party" atmosphere. I believe there is an OB resident who is responsible for the presence of mood-altering substances, whether or not he himself was actually present. I believe there is another OB resident of some prominence present. And I believe there may be a few others there as well also of some local significance. As they party, Shannan begins to succumb to the effects of something in her drink. Her voice on the phone sounds like benzodiazepines, barbiturates, or both. Maybe an opioid, but I don't think so. As she falls further under influence, I think the group there gets a little more aggressive in what they try with her and, whether directly or indirectly, she becomes aware that the risk of something grave is very real.
My guess is that as those who have a REAL interest in NOT being identified hear/see her calling police, they get out of there. That alone would explain Shannan's fear, especially in an altered mental status, of ALSO leaving - going outside where a group of her attackers just went. It would also explain why she runs from Pak, who otherwise should be her security and why she blames him for being in on it. In her head, he's trying to bring her outside where her attackers are.
I can qualify the hell out of this even more, but that's the crux of it.