I am going to analyze the information in the above links, and synthesize it into a scenario of the Stuart family's last day, when they disappeared. I will include local knowledge and reasoned observations. My effort will be aided by Doctor Occam, of Occam's Razor fame. In other words, the simplest of a choice of explanations for an event is the truth.
There are a number of theories advanced to explain the disappearance of the Stuart family. They are:
Someone had tampered with the Opel's fuel line so that it would eventually conk out and strand the family.
Mary fled her marriage because of domestic abuse.
Mary took off while lost in a fit of delusion.
Mary and her kids died of misadventure after abandoning her car.
Mary committed suicide, leaving the children to perish.
Mary and the girls was murdered.
The family was abducted by aliens.
We will consider all these possible theories as we develop the scenario below.
On Saturday, 10 December 1977, in the hamlet of Honeydew, California, at about 10 AM, 32 year old Mary Elizabeth Stuart loaded her two small children into the family station wagon. Mary was taking Jessie Flo and Fannie Fawn on a day of errands. It was chilly, at 35 degrees, though the day would warm to low 50s. However, there was no wind and there hadn't been any precipitation for at least a week. It was another fine clear day in the wilderness of northern California.
Mary planned to be home before dark; with about seven hours remaining daylight that day, it would be dusky by 5 PM. She was probably gun-shy of driving rugged Humboldt County's twisting narrow two-lane roads in the dark, even in the dry. Her small red German Opel station wagon, tags 4546-AII, was a distinctive and unusual vehicle (sample shown) compared to the giant 4x4s, pickups, and American sedans of the era.
https://upload.wikimedia.org/wikipedia/commons/4/46/Opel_Kadett_B_Kombi_BW_2011-09-03_13-38-00.JPG
On the day's agenda was the grocery store, laundromat, tv repair shop, and optometrist's office. Mary's errands were slated to take her to Fortuna, CA, about an hour and a half away,and/or Eureka, 20 minutes further north. (One source said Fairfield instead of Fortuna, but Fairfield is over a 9 hour drive from Honeydew. Nonsense.)
A map of her projected shopping route:
خرائط Google
Other than grocery shopping, the day's activities of Mary and her toddlers are unknown, with travel taking up at least three and a half hours of the seven hours of daylight. However, when the small German wagon didn't come home that night, Mary's husband, Byron McCray Stuart, didn't report his family missing. Nor did he call the cops on the 11th or 12th. On the 13th, a friend of Mary's filed the missing persons report.
While a search party of 25 volunteers searched the approaches to Honeydew, deputies interviewed Byron. Stuart was suspected of domestic violence and was a known drug abuser, which may or may not have accounted for his insistence that his family had been abducted by aliens. Given his reputation for hotheaded and erratic temper fits, his ongoing drug use, and his penchant for firearms, law enforcement was more than a bit skeptical. A neighbor verified Stuart's drug use and flakiness during the week following the disappearance, including raving about space aliens.
There is no information given whether law enforcement checked Eureka and Fortuna stores and shops for anyone who might have seen the young mom and her two kids on the 10th. They must have eaten, somewhere, sometime, but it isn't mentioned. The ongoing search failed to elicit clues.
The Opel was finally found abandoned on the 19th of January, 1978, on a logging road near her Honeydew home. Groceries were in it. Obviously, the little family had made it to town, then returned to Honeydew with the groceries. There's no mention of laundry, whether clean or dirty, nor a TV set, nor new glasses or contact lenses. There was also no sign of mother or children, nor signs of a struggle.
The little Opel was disabled by the steel fuel line's breakage, probably by the car bottoming out on rock. A severed fuel line stops supplying gas to the engine in under a minute, and the engine can't run until the fuel line is mended.
The rugged country around the stalled station wagon was searched, again with no sign of the Stuarts.
With the Stuarts' daily scenario complete, let's consider the possible theories of disappearance listed above.
First, if someone tampered with the car's fuel line during the journey, it would have quit running at the spot. At the site where it did quit running, tampering would be blatantly evident. Then, too, why would someone be lurking on an uninhabited road to cut a fuel line? And Mary wouldn't interfere with the tampering? Doc Occam snorts disdainfully at the possibility.
Now consider the flight from domestic violence theory. There's absolutely no evidence of it. Alright, is the cynical rejoinder, it went unreported, as domestic violence usually does. Fair enough. Maybe. But why would an abused woman leave the safety of the county seat, with its police force and services for abused women, to return to an obscure village just so she could bug out again? Nonsense, says Doc Occam. So, two theories dismissed.
There is no evidence Mary was deluded in any way. She was rational enough to drive, shop, and navigate that day—at least until she left the paved road on her return trip. However, there is no evidence she veered off her course either. So much for the third theory, that of a delusional runaway.
Then there is the theory of death by misadventure. A usual sub-theory is death by animal attack, even though a human is more likely to die by lightning strike than animal predation. However, as Doc Occam points out, why would Mary drag her kids into the wilderness in near-freezing weather, when they could retrace their route back down the logging road to pavement? Not that that would have been easy, given the rough footing in the dark of the moon, the temperature dipping, and the youth of the toddlers, but it's more inviting than the woods. However, neither remains nor signs of passage along the road or in the woods were reported.
Mary's death by suicide is also unlikely, for again, no remains, no signs, no predisposing symptoms.
This leaves murder as the only likely theory. There is evidence pointing to the most likely suspect—husband Byron McCray Stuart. And here Doc Occam agrees. When we search for the motivation for murder, as well as access to the victims, statistics (as well as the current facts) point at Byron Stuart. Most domestic murders involve a spouse.
A neighbor reported Stuart as being drugged and delusional for the week following his family's disappearance. His space alien fantasies were proof of delusions. Then, too, he didn't report his family as missing. There is no information that he even searched for them. There's no one else known to have a motive to harm Mary and her kids. And the Opel was found near his home, in a spot likely known to him.
Furthermore, there is no actual evidence that Mary and the girls were even in the car when it stalled out. There is no way of knowing who actually was driving the Opel wagon when it crapped out. But it was awfully conveniently located for Mr. Stuart. He might have even walked home from the Opel after he ditched it.
If we pay attention to Doc Occam, Mary Stuart and her daughters were murdered. Byron Stuart has since died. But the questions still asked are, Where are the remains? Did he have accomplices? Are they still alive to answer for their crimes? These are still open questions, and this is still an open case.
The Humboldt County Sheriff's Office is the investigating agency in this case. Their anonymous tip line is 707-268-2539. Murder should out.