Going waaay back to the TV show "Dallas," did it turn out that the guy who claimed to be Jock after Jock disappeared really was Jock?

No.  He was imposter.  He had been a member of Jock's crew in South America and became friendly enough with him to learn all kinds of personal details he could use.  And after he was injured and lost his memory for a time he actually thought he was Jock.  But it was eventually proven he was not.  At that point he packed and left southfork and the show.