Exactly.
And let's not forget,
this was the summer of the bicentennial. People were traveling like crazy all over and there were events going on everywhere. It would not have been unusual to see people on the road that summer. Also the Vietnam War had just ended and a lot of Vets had taken to the roads to travel around.
There was even a movie made around this theme back then
I believe these two knew each other and may have gone on a bicentennial road trip. The gun is key, and hopefully they can do some high tech forensics on it, which are likely much more advanced than they were in 1976 and figure out who had access to it.
From what I remember, the man stopped with the gun (Mr. Henry) was too far away from the area at the time of the murders to be a likely suspect. Previous information that I recall being posted (stated by Verna Moore, the now deceased Coroner), was that Mr. Henry's wife had been in the hospital in North Carolina and he had been there with her most of the time. Due to the distance, he would not have had the time to make a round trip journey to and from the crime scene in Sumter.
That, coupled with his polygraph results leads me to theorize that he did not do this, but may have known who else had access to this weapon and didn't want to disclose it.
It's also super important to remember that the authorities always hold back details of crimes from the public. So there's probably a lot we don't know so that their case is protected. I also think the Canada theory was focused on not only because of the alleged encounter at the campground, but because no one seemed to be looking for this couple. It was a lead that had to be ruled out, and it was. It made a lot of sense that they were from far away from Sumter, and as it turns out, they were.