Re: How would Sarek explain Spock 'Prime' to the other Vulcan survivor
First, time travel is no longer denied by the Vulcan science in TOS time.
Second, it would be easy to have him pass as a relative to Sarek (like an older brother). It's not like there is any paper trail of is existence/non-existence anyway since the whole planet blew up.
"I kept the Vulcan and married the woman, 'cause I didn't think Leonard would have it the other way around."