A few thoughts...I can't for the life of me figure out why no one from either Jock or Jane's family has even came forward to query the whereabouts of them. Even if Jock's father disowned him, the guy must have had friends, associates, co-workers that were concerned about him.
1. Jock seems to be very similar in a lot of ways to my dad. Back in the early 70s, my dad got out of high school and proceeded to hitchhike all around the US and Canada for a few years and even flew over to Europe for a summer of hitchhiking around. My dad worked in some kind of a plant in Atlanta, he sold books door to door in New York State as well as all kinds of things in between. With the ability of transients to move, crossing multiple jurisdictions, state lines and international borders in a short amount of time...
2. Where would one begin? Let's say for the sake of argument that everything said about Jock is true, his dad was a doctor and Jock was disowned. Jock didn't want to be a doctor, so he took off. He didn't keep in contact with his parents, but maybe he sent siblings a postcard now and then or called long distance collect to keep in touch. He stopped and worked as a teacher and then maybe took off on a hitchhiking tour. By the time anyone would realize that he was /gone/, he could have dropped off the face of the earth and ended up dead and buried in rural South Carolina.
3. Given everything else we know about the county dragging its feet in the investigation, I doubt highly that they made an effort to publicize the case. The media got a hold of it and it was news across South Carolina, but if the parents or siblings were looking far away in Canada... With no Internet and little communication at that point in time across jurisdictional lines, the search would be very hard and time consuming.
4. Maybe the family just gave up and decided to let it rest. The other big case I follow here at WS is the Lyon sisters of Maryland. Their parents are described by people who know them as interested in finding out what happened to their daughters, but also having moved on with their lives.