In re to your last sentence, it was never determined that "Jock" had an accent. I don't know if LE never thought to ask the KOA guy who claimed to have met "Jock", or if this information was just never reported to the public, but as far as I know we do not know whether "Jock" had an accent or not.
This possible connection with the Dirty War is interesting, but the fact of the matter is that a local man, Lonnie George Henry, was stopped for a DUI and tests found on a gun under his front seat proved it was the murder weapon. He admitted he had filed the serial number off the gun in the previous six months. This, to me, indicates that Lonnie George Henry, a local drunk and ne'er do well, or someone very close to him like a family member, someone who had access to his gun and was able to 'borrow' it and then replace it, murdered this couple. So to me at least the issue of who killed this couple isn't so much a mystery - it's who they are that is the mystery.
So the murderer was most likely Lonnie Henry or one of his family members. Again, all of these theories are interesting but I'm thinking it's much more simple than that. This couple were outsiders. They were travelers, so either they did or were probably assumed to have had considerable cash on them. IMO, Jock and Jane Doe were robbed by a member of Lonnie Henry's family or someone close to him (or less likely, Lonnie Henry himself, since he was purported to be at a Monroe, NC hospital visiting his sick wife at the time of the killings).
I don't think we will find Jock or Jane Doe on any missing person's site. Although I wasn't born yet, from what I have heard and read the 70s were a very different time. This young couple may not have had the best relationships with their families, and who knows how long it had been since they had been in touch with their families even before they were killed. The families probably assumed they had just gone and started their own lives somewhere else, and maybe never even filed missing person's reports. I do believe they have family out there - cousins, siblings, nieces and nephews - but I don't believe these people are actively looking for Jock or Jane. And let's face it, we have heard about this couple because we are Websleuthers and follow missing people and the unidentified cases, but if you walked up to 100 random people on the street (outside of the SC area where this happened, of course) and asked them if they had heard about this case, I'm sure at least 99 of them would say no, they hadn't. So Jock and Jane's surviving families, wherever they are, are probably not looking for Jock and Jane, nor do they know we are looking for them, or that this case even exists.
JMO