The stew is the vital key concerning this as it confirms what Betty Jane said when Lucy last talked to her, stating they were going to be back shortly. If they were to be back shortly, then they must not have intended to go very far out of the area. Something happened from that time and to the time of their intended "plans" that were to be short. But, you might be right, that "someone" could have been a hitchhiker, possibly a clean-cut one.
Did they stop to get gas? Did someone at a gas station or corner store see De Mattos at a counter with large amounts of money in his wallet? I don't think his car would have broken down, because it would have been found and seen for a longer duration of time. Perhaps they were held at gunpoint as the gunman sat in back and ordered them to a certain destination outside of Honolulu. But then, no reports were ever received of sightings of the car or them in it.
Here is an in depth look at a 1957 Pontiac Star Chief:
1956 Pontiac Star Chief Survivor - Runs and Drives Well! - SOLD! - YouTube
Hard to believe this many car thefts on an island.
http://archives.starbulletin.com/2001/05/31/news/index.html
Maybe they did go get gas and someone saw he was carrying a lot of money. December 9, 1966 was on a Friday. Perhaps they were running errands and preparing for the weekend as they went to the laundromat early that morning. I wonder if Betty kept a journal or diary??