Josie's cause of death cannot be determined, according to a discussion between Hawk and Cooper in ep 24:
"This is worse than the bureau, all the international documentation. Eckhardt... Josie... (reaches for a folder) Have you seen Doc Hayward's autopsy on her? Says he couldn't determine cause of death. And the body weighed sixty-five pounds."
Later (ep 28), Cooper tells Truman about his vision of BOB and LMFAP:
"She was trembling with fear. I would go as far as to say quaking like an animal. I might venture a guess to say it was the fear that killed her. At the moment of her death, I saw BOB. As if he had slipped in through some crevice in time. Upon reflection, I believe there's a connection between his appearance and Josie's fear, as if he was attracted to it. Feeding off it, somehow."
So it does seem to be "fear" that the series relies on to explain the death. But it is by no means "simple," as you say. In any case, I think the guy who wrote the Mill Conspiracies essay is a pretty trustworthy guy... so you can rely on him. ;)