I hate to say this, but the only unresolved part of this mystery seems to be what may have happened from when Diane was reportedly last seen to the time she presumably died and maybe her cause of death as well. I say that because, from what investigators know, her body has been identified with evidence that suicide was her cause of death. It sounds like the results of DNA testing of said body, for one, are conclusive. That would mean Diane's family is in denial, which is absolutely understandable given how close her family members seemed to her. I extend a lot of empathy to them in that it's extremely difficult and even frustrating to come to terms with something you never could've imagined happening to you, your family, or someone close to you in general.

All said, I think it's probably easy for Diane's family to assume that foul play was involved, especially since it seems like there was some long-standing tension with Bennie. After all, he was presumably one of the last people to see her alive, and he would later reject his marriage with Diane for no apparent reason. It seems like we don't even know how he felt about his upcoming status as a parent.

Speaking of which, it seems like Diane was insistent that she and Bennie would raise their unborn child together. That could have been something she was even especially happy about or looking forward to, which would likely have made a miscarriage (if Bennie was telling the truth about such) all the more devastating. A miscarriage is one of those things that can change people in ways beyond what anyone can predict and in a very short amount of time at that. Therefore, I wouldn't be shocked if that could have resulted in a mental breakdown or something similar in Diane's life, hence suicide being a factor to consider beyond the physical evidence such took place.

Overall, I wouldn't absolutely put either foul play or suicide aside. I think it's hard to say whether or not Bennie was truthful, and I see the possibility that Diane (at least as far as Bennie knew) was supposed to go to her parents but decided to isolate herself instead, especially if to grieve a miscarriage. Ultimately, I think it's clear that it could be helpful to know how Bennie and Diane were doing individually and as a couple around the last time they spoke or saw each other.