This one's so bizarre.
First off, I think the fact that the case file says these men were "mentally challenged" just reflects the time period. A lot of cases that involved mentally ill or developmentally disabled people use terms like that. Because Gary Mathias attended this program, despite being schizophrenic and apparently not developmentally disabled, IMO the program seems like it was more of a catch-all for troubled young men. Plus they were able to live at home, which makes me think that they just needed some extra structure, but they weren't struggling so badly that they needed to be inpatient somewhere. Maybe the program taught them work skills or provided opportunities to get a high school diploma or take college classes.
So the man who said he saw men with a woman and a baby (in one of the Charley Project articles posted by abra0313) had had a mild heart attack prior to this sighting. I have 2 thoughts regarding this - one being that this sighting never happened at all, or that he didn't see what he thought he saw. I don't know anything about heart attacks specifically, but heart problems in general can cause dizziness and confusion, which might be the case there. My other thought is that the man did in fact see these men with a woman and a baby, and that it was some sort of ruse to get them out of and away from the car. I feel like that would be a perfect way to lure people from their car - call for help, say your baby is sick, etc. plus it's cold outside and I feel like people have a natural instinct to protect children. Maybe they stopped because she was in the road, she said her car was stuck/broken down, got them off the road, and then... I don't know. I can see how they might've left the car, but the rest gets me.
I don't think it's feasible that the men who were found were able to walk 20 miles in the cold/snow, not to mention in the dark. Obviously we don't know what they were wearing, but if it was somewhat consistent with Gary's clothing description, I don't see how they would've made it, or why they wouldn't have turned around and headed back in the direction of the car. The unused food and propane is baffling as well. How the bodies were found almost seems staged to me, but I can't think of an explanation for why that would be the case either. And I don't get how Gary couldve escaped the same fate, since it seems like they were all in the same situation. I would hope that they've scoured the area for bones or clues in the years since this happened, so I assume he made it out of that particular area, at least.