It's certainly hard to take what ANYONE says as fact without personally knowing that person. However, I've long thought this about DC. The fact that they've been together for so long, and doing exactly the same as, or very similar to, what ED was doing at the time of her disappearance, give much credence to his assessment of the situation. He knew her like hardly anyone knows another person, from what I can figure. I know that he's given some indication as to why he thinks she could have been a victim of foul play, and I also think those are valid points, but there is just the fact that that's his assessment -the closest person to the situation. I also add to this the fact that her mother seems to support other things he's said, such as comments on her nature and habits, that I have to think he and his ideas about the case very reliable.
I'll add this for contrast: There was a woman who went missing in the desert in the States. She and her husband were older, and she went missing during the summer. The husband came up with what seemed like a very strange story -that someone had kidnapped her and taken her to...somewhere. Can't remember. But he was very specific. Now, he seemed from the beginning to be a less reliable witness for several reasons -he hadn't done the "re-tracing" of route like DC has; they hadn't been joined at the hip living this extraordinary lifestyle for the last many years, etc. In the end, it looks like she got disoriented or suffered some medical event and succumbed to the desert heat. Her body was found near where they were separated.
DC's theory, on the other hand, makes me sit up and listen.