Re: John Christopher a Regular?
The other problem with the scenario is you have the Enterprise stuck at Earth in 1968 for X number of episodes with no place to go and investigate. They're cut off from Starfleet and any repair facilities. They can't go to Vulcan or anywhere else without contaminating timelines.
Curly: Moe, Larry, the cheese! Moe, Larry, the cheese! (Horses Collars, 1935)