I have just read this thread for the first time. A few thoughts come to my mind about the case.
The visitor appeared at the base of the mountain and led LE back up to where he stated that he had last seen Barbara. Clearly, he had been up the mountain and knew the trail. If he had come to the area for the first time, how would he have known the trail if Barbara had not brought him up it to begin with? In my mind, a scenario where he did something to/with Barbara BEFORE going up the mountain is much less likely than other theories.
An animal attack is possible, since Bears and Mountain Cats exist in the area. But such an attack would most certainly have left evidence behind. Clothing, personal articles, blood, hair, signs of struggle, screams, etc. Also, the fact that this was a well traveled trail and the fact that they would have gotten there quite late in the morning would make an encounter with an animal less likely.
The statements in previous posts which indicate that there was a lot of loose shale and that the terrain was very steep lead me to believe that a slip and fall is a very likely possibility. Going DOWNHILL is much harder than going up, especially in these conditions. When you go uphill, you can see more clearly where you will be stepping - and if you slip, you usually fall forward and can catch yourself with your hands and your toes. If you are walking down hill, and slip, the fall is backward, and you have nothing to break your fall with. Your heels have nowhere near the flexibility and traction that your toes and feet do. Loose shale makes going down hill very treacherous.
A few years ago, the body of a soldier was found in Adak, Alaska. He had been missing since World War II when he had failed to return from sentry duty. He had actually fallen into a crevice and remained unseen and unheard, in spite of the large scale search which was made for him. I wonder if Barbara may have met a similar fate.
Scenting Dogs have been mentioned. I have trained a number of dogs in both tracking and in searching for objects and persons. You cannot come to any conclusions based on the very sketchy information provided in the articles posted.
There are a number of factors which enter into a dog search. First is the ability, experience, and training of the dog and the handler. Tracking or Search and Rescue are skills which take a lot of teamwork and training between dog and handler. A run-of-the-mill K9 patrol dog going into the area may or may not be able to find anything. A fully trained and qualified tracking dog, GIVEN THE SCENT, would take you right to the subject.
I can state from personal experience that sometimes you arrive at a search scene and those running the search have no idea of the capabilities of tracking dogs or their optimum deployment. They may simply order you and your dog to a sector with a lot of other people and give you no starting scent at all. (Given nothing, do everything!)
We do not know the experience level of these dogs/handlers, or whether they were given any scent, or where they started looking, or how long it took to get them into the area, or what the weather, temperature, wind, and rainfall was. All these factors play a part in any search.
Furthermore, the ABSENCE of evidence - in this case the statement that the dogs did not find anything - cannot be used to prove or disprove anything.
Barbara's revolver could possibly provide a clue. If the make, model, caliber, and serial number were to be entered into the national stolen gun registry, it might turn up at some time. IF this were to occur, then it would be obvious that she was murdered and her handgun stolen.